Please wait for some other replies from experienced people, but my understanding is this (and I have never done a migration!)-
-If you change plans (to the Lightroom 1TB Plan) then you are effectively cancelling the 'Photography Plan' but Lightroom-Classic should still function but in a reduced capacity- (see this Lr-Queen link), but your Catalog file would remain.
https://www.lightroomqueen.com/cancel-cc-subscription-photos/
-The instructions for the Migration Tool seem to suggest that you do not need Lr-Classic running- you only need to select the Catalog (.LRCAT) file to migrate (and the photo files, of course!).
- The Help page also suggests that you have the Classic Catalog in a 'pristine' state using Lr-Classic to fix problems like 'missing' photos, etc, etc, BEFORE you attempt the migration.
https://helpx.adobe.com/lightroom-cc/using/migrate-to-lightroom-cc.html
https://helpx.adobe.com/manage-account/using/change-plan.html