I think the short answer is “it depends.” 
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Any device which currently shows “placeholder” in the IDE is already using the new architecture, and should be fine. That should include cloud to cloud integrations and devices already using edge drivers.
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Hue is a special case because there are two available stock integrations: cloud to cloud and LAN. if it’s cloud to cloud, you’re fine. If it’s LAN, ST is currently working on a stock Edge Driver version, but it’s not available yet. There is a community-created version by @blueyetisoftware , but you would have to subscribe to that and manually transition. See the link to the quick browse lists at the end of this post to find this and other custom Edge Drivers.
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many hub-connected devices which currently use stock Groovy DTHs will be automatically transitioned to stock Edge Drivers, but not all. SmartThings has made the decision to NOT transition devices which have been officially discontinued by their manufacturers even if the devices have been running fine with stock groovy DTHs for years. This will obviously require research.
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any device which adds to ST as a “thing” and which then had to be manually changed to a stock DTH probably will not be automatically transitioned. More research.
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any device which used a parent/child structure, even if it was with a stock DTH, is probably going to have some glitches after the transition. Voice assistants might not be able to control the individual children (now called “components”) any more. The individual components may have weird names and may not be able to be renamed. Routines referring to the child devices may break.
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IFTTT and other thirdparty integrations like Logitech’s Harmony which used a groovy smartapp may break. Some of these third parties like IFTTT are working on new integrations for the new architecture, so it’s just a question of if the new version is ready in time. Other companies like Logitech have decided they are not going to create a new integration, so it will be up to the community to create one, if possible. So more research.
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voice assistant should be ok except for the multicomponent issue described in 5. There is a workaround using virtual switches, but it has to be manually set up.
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SharpTools should work fine with the possible exception of multicomponent devices. Not sure about ActionTiles.
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As @jkp notes below, @rboy is waiting on SmartThings to complete work on the new platform, so he doesn’t have a lot of details yet. Here’s his Migration FAQ page, so watch that for updates as more info becomes available:
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@yvesracine will not transition his products to the new ST platform, but continues to actively support the Hubitat versions
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the new architecture does not presently display network IDs, which are required for setting up zwave associations and Zigbee groups. So even when the manufacturer is providing a custom edge driver, you might lose some functionality. @lmullineux just this morning released a new utility to allow you to view these IDs and @Mariano_Colmenarejo has a zwave Edge Driver that lets you set associations, but that’s all a lot of manual work. (You can find both of these utilities on the quick browse lists under Miscellaneous. See the end of this post.)
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it appears that some of the stock Edge Drivers reset existing zwave associations and parameters, in some cases incorrectly. That could cause lost or broken functionality, but it’s hard to predict in advance. So more research if you currently have zwave devices using association or you have changed any of the default parameter settings.
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if a device uses a custom DTH, it will be automatically transitioned to a custom edge driver with the same fingerprint if that custom driver has already been downloaded to your hub. Otherwise the system will try to guess the best matching stock driver. This could result in lost or broken functionality. Routines might also be affected.
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no details yet on what will happen to virtual devices and the routines that reference them. There are several possible variations: you do or don’t have a hub, you created the device with the wizard in the android app, you created the device through the IDE. But while we know these variable may be important, we don’t know the exact impact.
There are some community-created edge drivers for virtual devices that work fine if you want to go ahead and manually transition to those. You can find them on the Virtual Devices Quick browse list. (See the end of this post.)
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Devices currently using a custom edge driver are supposed to be left unchanged by the automatic transition.
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It’s unclear what will happen with some individual models of Leviton and WeMo WiFi devices: they don’t all seem to be included in the cloud to cloud integrations.
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You are limited to a total of 200 devices, 200 local automations, and 50 edge drivers.
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Webcore will not be automatically converted and will stop running eventually. If you use Webcore, you have a lot of research to do. Quite a few people are switching to the thirdparty rules engine from SharpTools. It supports a lot of complexity and doesn’t count against your ceiling for automations. But other people will make other choices.
19, The ABC button controller will stop working once the transition is complete. So will any groovy weather smartapps. @taustin has a weather service edge driver that will work for some weather use cases. See the Quick Browse list for Weather at the link at the end of this post.
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The old platform only required a mobile device for access to administrative tools like the IDE. The new platform will require a pc for most similar functions, even device logging. Anything you see that references the “CLI” is referencing a program you have to download and run on a pc or Linux device.
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SmartLighting is going to change, but no details yet or if there will be an automatic transition or what might change.
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If you have downloaded more than one custom edge driver for the same device, the system will randomly pick one. You can then manually change it to a different one.
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There is a table in the community-created wiki listing edge drivers by device model, but it’s brand new and a lot of stuff is missing. If you don’t see your device listed, it doesn’t mean there isn’t an edge driver, it might just mean no one’s made the table entry yet.
https://thingsthataresmart.wiki/index.php?title=Table_of_Edge_Drivers
- If you have a V2, V3, or Aeotec WASH hub, just check to make sure the firmware is up-to-date and it should be ready for the automatic transition.
The hubs that act as Wi-Fi mesh routers, on the other hand, continue to have some issues so that’s a little unclear.
Samsung Connect Home/SmartThings WiFi Hub Firmware Release Notes - 0.43.05
- The original announcement said the groovy cloud would go away on September 30th. That has since been quietly changed a couple of times. First it was changed to say the IDE would be removed October 15. Then it was changed again to say the transition would BEGIN happening on September 30. So just because a device/integration/smartapp works on October 1, don’t assume it will continue to do so. It may just not have transitioned yet.
Here’s the official announcement thread, in case it changes again. 
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So…”it depends.”
To find community-created edge drivers, see the quick browse lists in the community-created wiki.
https://thingsthataresmart.wiki/index.php?title=Quick_Browse_Lists_for_Edge_Drivers