I am converting my existing zwave devices from an older URC hub (THE worst platform ever) to ST. Overall the conversion has gone very well. I have about 60 devices and 54 of them converted over great - it was actually really easy once I figure how to get around a few of the quirks of the ST pairing process (exclude the device first, etc). Anyway, I have 6 Lutron switches that refuse to enter into pairing mode. When I click the switch to enter pairing mode (just like all the other switches I have) then nothing happens and they do not enter into pairing mode. These switches were previously paired to the URC hub so I know they work. I am at a loss - any ideas on how to force my Lutron switches into pairing mode?
Actually, your Lutron switches were never “paired to the URC hub.” The URC hUb does not have a Lutron radio inside it. ( it does have a Z wave radio.)
Instead, you established an LAN connection between your URC hub and your Lutron smartbridge pro device. Your individual Lutron switches were paired to that smartbridge pro device.
In order to use the MX HomePro Caseta two-way module with your Caseta system, you are required to enable Telnet support on the Smart Bridge Pro and send the Integration Report. To do this, you’ll need to do the following using the Lutron app:
http://www.urccontrolroom.com/_media/mxhomepro/files/mx_homepro_advanced_devices_integration.pdf
The SmartThings integration works exactly the same way. 
The SmartThings hub does contain a Z wave radio. It does not contain a Lutron radio. The official integration requires a Lutron SmartBridge (it can be the regular model or the pro model) and then you follow the directions to integrate the SmartBridge device with your SmartThings account. But the switches are still paired to the Lutron SmartBridge. They do not get individually paired to the SmartThings hub.
This should work whether the Lutron SmartBridge is still integrated with the URC hub or not, it will accept multiple integrations of this type. 
So you really shouldn’t have to touch the Lutron individual switches at all as long as they are connected to your smartbridge device. If you already deleted them, just reconnect them to the SmartBridge following the standard Lutron instructions and then follow the instructions in the SmartThings support article to connect the Lutron SmartBridge to the SmartThings hub.
Again, Z wave devices are completely different because in that case they do pair individually to the SmartThings hub. And that was true for both the URC hub and for the SmartThings hub.