According to the migration guide, upgrading owncloud to nextcloud looked very straightforward and easy. However, it’s not the case. First the downloading fails, but managed to solve this with a suggestion this forum. Next upgrade fails with the following message: Exception: Updates between multiple major versions and downgrades are unsupported.
So far I have not been able to find a solution for this. If anyway figured out how to fix this, please let me know how.
OwnCloud 10.2.0 > Nextcloud 12.0.12 (as suggested by migration tool)