I usually do the following
I'd recommend installing version 19 on the new PC first.
Get a copy of the library backup onto the new PC. Restore the library and check to import the settings.
Reconfigure any audio settings and file paths that might need to be changed, including the auto import settings.
Then select all the tracks/media in a file view(s). Use the Rename,Move fonction under library tools and repoint the database to the new path(s) (if they are not identical, different drive letter etc.)
Make a library backup from the new PC.
Run autoimport manually just to check.
Should be good to go.
After checking that everything is working fine. Install version 20 (don't remove version 19 yet). I usually wait a bit before actually deleting the prior version. Just make sure if you are running media server, to not autoload the version 19 files.