Maybe that can help someone, who doesn't want to wait for version 10.
I successfully migrated from ownCloud 9.1 to nextCloud 9.0.53 on a shared host (All-inkl).
I cannot garantee, that this will work on another system. I just describe, how I migrated.
ownCloud is installed in a folder /htdocs/cloud/9.1.0/owncloud
Datadir is in a folder /htdocs/data/owncloud/data
Tempdir is in a folder /htdocs/tmp/owncloud/tmp
PHP 5.2.17 as Apache module
Steps to reproduce
! Prevent synchronizing eg. by stopping ownCloud synchronizing app on local PC or by changing the ownCloud URL to a maintenace page. Now or later before copying the database and the data directory.
! Maybe it may be safe to disable all owncloud applications before executing the following steps. I didn't.
1. Extract nextCloud distribution to /htdocs/cloud/9.0.53/nextcloud
2. Create /htdocs/data/nextcloud/data
3. Create /htdocs/tmp/nextcloud/tmp
4. Create an new nextCloud-DB
5. Copy /htdocs/cloud/9.1.0/owncloud/config/config.php to htdocs/cloud/9.0.53/nextcloud/config/config.php
6. Edit /htdocs/cloud/9.0.53/nextcloud/config/config.php
'datadirectory' => '/htdocs/data/nextcloud/data',
'tempdirectory' => '/htdocs/tmp/nextcloud/tmp',
'version' => '126.96.36.199',
'dbname' => 'nextCloud-DB',
'dbuser' => 'nextCloud-DB user',
'dbpassword' => 'nextCloud-DB pass',
7. Set permissions for folders and files (wwwroot as user)
! At least now client synchronising should be prevented as described before.
Execution of the migartion:
8. Make exact copy of the owncloud-DB to a new nextCloud-DB
9. Copy all files from /htdocs/data/owncloud/data to /htdocs/data/nextcloud/data
10. Point host configuration from htdocs/cloud/9.1.0/owncloud to htdocs/cloud/9.0.53/nextcloud
11. Open nextCloud in browser -> Update starts and should complete without error (I had just warnings of apps that have been disabled)
12. Login in to nextCloud
13. Enable needed applications
14. Start synchronizing (Client didn't recognise the changes server system in my case -> synchronizing behaves as usual)
After these steps, which I reproduced on three instances, I did not get one entry in the log file.
I made a complete copy of the database and the data directory instead of a backup. In case of an error, I would have been able to return to the previous installation by just changing the host entry to the old value.