I had naively assumed that there won't be much difference between one nextcloud (provider) and another. But when I was recently forced to move to a different provider - surprise, surprise - from this moment on sync has been much faster, and this despite the new 'cloud' being free. Since I saw questions about this a few times, here is how migration worked painlessly.
made a JEX backup
check the 'fail safe' flag is set
downloaded the complete Joplin folder (nextcloud turns it into a ZIP container)
unzipped
NOW
here is what you need to know for all current nextcloud instances : nextcloud has no button which allows you to upload a folder, the '+' button only allows upload of single files. Therefore open your nextcloud in a desktop browser, and drag-n-drop the Joplin folder just created (unzipped) into the home directory.
patiently wait for the upload to complete
change the sync target in your primary Jop app and sync
Voilà !
So a .jex backup will contain everything needed for Joplin to sync from/to?
Since I have my own nextcloud it should be really simple to upload the backup and drop it into the right folder. But it's not at all clear from the docs whether what's stored on the syncing device is a straight copy of the joplin storage on the app device -- which would be a backup of the raw directory -- or whether it is in some special format like .jex. So I never know quite what to copy for these experiments.
@seatrout
misunderstanding !! I meant make a local backup of Joplin (in case sth goes wrong),
and THEN copy the Joplin folder (not local this time) from nextcloud 1 to nextcloud 2. Hope this makes sense, it worked for me. In this case the file structure etc. is obviously exactly what is needed for nextcloud.
check the 'fail safe' flag is set (for later, just in case)
download the complete Joplin folder from your OLD nextcloud account using a browser / web I/F (nextcloud turns multiple files into a ZIP container), this is a desktop / cloud file operation, outside Joplin.
unzip it on desktop
( NOW here is what you need to know for all current nextcloud instances : nextcloud has no button which allows you to upload a folder, the '+' button only allows upload of single files. Therefore open your NEW nextcloud web interface in a desktop browser, and drag-n-drop the Joplin folder just created (unzipped) into the home directory )
patiently wait for the upload to complete,
change the sync target in your desktop Jop app and sync,