All in all, the migration from ownCloud server to nextcloud server was kind of easy. 
But I find the client migration quite hard. As far a I observed, ownCloud clients won’t connect to nextcloud on Windows and on MacOS. I didn’t expect that, because they both use Webdav, what changed?
So on my Mac system I went to newly install the nextcloud client and do the whole setup. I’ve chosen my old owncloud directory as sync directory and was hoping it would recognize the files and not resync everything.
However, after a lot of checking, the client wants to resync 74 gb to my nextcloud that is already on the server. With my current internet-connection this is simply impossible. 
What should I’ve done differently to get a seamless migration for the client?
–– Thomas
Okay, nevermind, the old client works again! Maybe I’m just stupid or it randomly didn’t work before… 
In this case: This is a question for the future. If OC and NC develop some much difference that sync clients won’t be compatible anymore – will there be an easy way to migrate to a different client without resync?
–– Thomas
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I just tried the obvious solution for this:
Copied the contents from $HOME/Library/Application Support/ownCloud to $HOME/Library/Application Support/Nextcloud. (While both clients are closed.) Then reopened Nextcloud.
It seems to work without side-effects. 
The clients are probably quite similar, apart from the “branding” – but I’m not giving any guarantee on that trick 
–– Thomas
Thanks! I’ve just tried this method and it works pretty well! 
I would just add that it’s required to rename the file owncloud.cfg into nextcloud.cfg in order to make the settings available to the Nextcloud client. After this operation all folders were correctly linked and no need to resynch everything.
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