I'm looking for recommendations on how to proceed. Which should be done first? Transition LR to CC and then change OS's or other way around?
Doesn't really matter.
OK, fair enough.
Regarding the OS change and external drive incompatibility with Win10, should I format a new drive for Win10 and just copy the MacOS Extended drive contents to new Win-compatible drive? Other way of doing it?
So far, I've copied the photos from the Mac OS compatible drive to another drive formatted for Win10 access with the same file structure.
Can I move the old .lrcat file to new computer and rename the windows drive the same as the original Mac OS drive to complete the migration? Any problems to see with that?
Copy the catalog file to the new computer. As far as I know, you cannot rename a Windows drive to be the same as the Mac Drive, Windows uses letters rather than names.
The process for updating the location to your new Windows formatted drive is not difficult. In Lr under Folders, you'll see a question mark, click on it and update the folder/drive location to the new drive. If you've retained the same folder structure on the new Windows drive you shouldn't have a problem.