I’m aware there is already documentation on migrating a NC instance to another server. That takes care of moving the data, but that’s only half my problem.
The other problem is I have many folders that I have shared to third parties. When I move domains this will break for users that were either logging in via the web based interface or who had their own NextCloud servers that mounted the shares.
Is there a way for NextCloud to notify either via push or email that the user’s share has migrated to another domain, and to click something to update their mounted share automatically if they had their own NC instance? This might be more something appropriate for a feature request.
As far as I know there is no push service. You can create permanent redirect to the new location. The clients should update to the new address. Shares in theory as well, but I never checked if they really did (and you have to use them). For that you need to keep this redirection active for quite some time.