If the lock is still using a default or custom DTH, then SLGA isn’t going to work for programming codes. You’re going to have to migrate it to an Edge Driver. It should be migrated by the ST team at some point, but if you need to make code changes now, you will have to do it yourself.
First, while your lock will be supported by the stock ST lock Edge driver, there are some bugs in that driver and some missing features. I would suggest installing the beta version of the Z-Wave Lock PH driver from @philh30. In particular, it fixes a problem with lock code names being reset periodically and it exposes lock codeName for use as a trigger in Routines.
Next, if you have Routines that are using your lock, you need to decide if you can live with those Routines being deleted if you exclude/include your lock to switch the driver. If not, there are a couple of options to keep them from being deleted such as creating placeholder virtual devices you can add to the Routines temporarily.
In order to migrate from the custom DTH to the Edge driver, you will need exclude your device from your network and then go into the IDE and delete the custom DTH. Once you do that, you can add your device back to your hub and it should pick up the Edge driver you installed. You can check if the Edge driver is picked up by going into the device’s detail panel, click on the 3 dot menu, and see if the Driver option appears. If so, then it’s on Edge and you can see which driver is installed and switch drivers if necessary.
Your programmed codes are still going to be in your lock and will not be deleted and will still work. When you first go into SLGA, the code names will be based on the slot number, i.e. Code 1, Code 2, Code 3. You can give them friendlier names that will stay in SLGA unless you change drivers as the code names are maintained by the driver.
Hopefully this doesn’t sound too complicated and feel free to follow up here or via DM if you need any assistance.