There is not yet an official fix. However you could try to manually adjust the version string in your config.php, as Jason started above, or discussed here, after full backup, especially database dump:
There is no official repository from Nextcloud developers planned, but here you find a list of repos from other users or the official distro maintainers:
hi there,
the so easy migration is not so easy as it could be. The migration page claims:
This is the compatiblity overview:
ownCloud 9.0.x is compatible with Nextcloud 9.0.x ownCloud 9.1.x is compatible with Nextcloud 10.0.x ownCloud 10.0.x is compatible with Nextcloud 12.0.x
Iām stuck on a ownCloud 10.0.2, the latest stable release from oc.
When I put in Nextcloud 12.0.0, and try to run the upgrade with āphp occ upgradeā, I get this:
Exception: Updates between multiple major versions and downgrades are unsupported.
Update failed
Changing values in the config.php does not appear to be solution, still I get the same error.
Of course, at some other place itās written:
Note that ownCloud 10.0.1 can not yet be migrated to Nextcloud 12
Nextcloud - Migration guide
But why there is a list claiming the opposite? 
Soon 12.0.1 will be out which will 99% sure support moving from 10.0.2 to 12. One or two more weeks at most 
EDIT: Nextcloud 12.0.1RC4 - HELP TEST! -> 12.0.1RC4 is now in testing, you can help out. Yes, it took long - we made a LOT of improvements.
Soon 12.0.1 will be out which will 99% sure support moving from 10.0.2 to 12. One or two more weeks at most
jospoortvliet - Marketing & PR
that sounds good, thank you! 
Until that, just have a look on the answers above yours ;).
Is there now a save way without a lot of handcraft work from the 10.0.2.0 to NC?
Will it work, when I follow https://nextcloud.com/migration/ and then change the repo http://repo.morph027.de/#nextcloud-server on my Jessie-Box?
Hmm the migration page still sais:
Note that ownCloud 10.0.1 can not yet be migrated to Nextcloud 12, we are still testing this upgrade path to make sure it is 100% reliable!
So I guess nothing changed about that so far. But I would also see some priority in that since an easy safe migration is still the one of the most important thing bringing people to use Nextcloud. I wonder what the reasons are that this take so long or if it is already solved for coming minor release. I will have a look at github.
The minor release of NC 12.0.1 is a bit delayed, so probably all other milestones are delayed as well. If you are not already on NC 12, I would wait for this minor release. If you need to get away from ownCloud urgently, you still need a bit of manual work, in many cases it should be acceptable to wait another 1-2 weeks.
Here is the migration bug report on github:
Nextcloud 12.0.1RC4 - HELP TEST! -> Nextcloud 12.0.1RC4 supports upgrading from oC 10.0.1 and 10.0.2, I tested it and we can use some more testing help!
It should be out in a few days, provided no issues are found.
Today I migrated from OC 10.0.2 to NC 12.0.1 in a testing environment with success. I had dared to do so, because the annoucement of NC 12.0.1 promised to have fixed the migration issue.
Please note:
I have not been able to use the migration tool, since it still said, that there was no upgrade path from OC 10.0.2 to NC 12. So I took the manual way of migrating as decribed above.
Is now an upgrade path from 10.0.3 available?
There hasnāt been a new version since the end of September, so probably not yet. NC 13 should probably work (expected end of this year). If you can set up a test system, it would be great to run some tests with the beta version: 85% faster LDAP, 10x read/write speed of Windows Network Drive: Welcome to Nextcloud 13 Beta but donāt use this in production yet.
I am very confused!
Iām trying to migrate from ownCloud 10.0.3.3 to Nextcloud 12.0.4 but get the error:
Current version is 10.0.3.
Could not determine migration path to Nextcloud.
I have been waiting for Nextcloud 12.0.4 as I understood this would support migration from ownCloud 10.0.3.
Am I wrong or am I doing something wrong?
Thanks,
Cliff
Iām exactly in the same situation (owncloud 10.0.3.3), I have been waiting for the newer version during 3 months, I had understood on some forums that we must wait for the 12.0.4ā¦
The file I have uploaded from this link for the migration seems to be not up to date : https://download.nextcloud.com/server/installer/migrator/index.php
Is it the case ?
There still seems to be a bug somewhere, ref:
Hi,
Is there any progress on this? Iām stuck in limbo at the moment - I want to migrate to Nextcloud but canāt because the migration script doesnāt work, and I donāt want to upgrade my ownCloud instance because migration from ownCloud 10.0.4 to Nextcloud doesnāt seem to be supported.
Thanks,
Cliff
I am not sure if a manual upgrade could work. Make a backup before, then:
data/ and config/-folderIf you are lucky it just works.
Thanks for the suggestion but Iām not comfortable with ānot sure ifā¦ could workā and 'If you are lucky it just works." I would rather wait for this problem to be fixed.
Thanks.
I said luckily because I havenāt seen any feedback on the upgrade process itself. But the issue you are running into is just that the migration path is not detected correctly. The manual upgrade is probably working quite good, and with a complete backup the risk should be manageable. But it is ok to wait for the issue to be fixed.