On the licensing we got asked if we sell Claris Pro/Server plugin separately. What a terrific business idea! But no, we don't ask for extra money to move from FileMaker to Claris. Just make sure you have a recent license, which includes 12.5 (or later), so you can use our current plugin versions. And eventually most of our customers may move in a few years, when the Claris product line has a more attractive offering compared to the FileMaker line.
We'll update documentation over the time to add Claris instructions, but so far just the folder names changed. If you install by script, the correct path is used automatically. And if you manually install, you probably have no problem finding the Extensions folder.
Let us know if you have questions or trouble.