Add files manually
Recently I had to merge some clouds the "hard" way. Point was moving all files without down- and then uploading again. Using ssh it's quite simple.
- copy all files to new users folder in your nextcloud data directory
- ensure permissions are correct: hown -R www-data:www-data <path/to/new/files>
sudo -u www-data php occ files:cleanup
sudo -u www-data php occ files:scan (--all or specify user id; takes a while on large instances)
- Confirm everything is where you want to have it