How to inherit sparsebundle on macOS Server Time Machine service?
As it turns out, it is super easy with a single Terminal command.
First, you need to mount the Time Machine share using Finder. Just browse to the server, then to the share so that it mounts. I mounted with Admin privileges since Admin owns the the Time Machine sparsebundle.
Then, you open Terminal and run this command:
sudo tmutil inheritbackup "/path/to/backup.sparsebundle"
For example, mine would be:
sudo tmutil inheritbackup /Volumes/Time\ Machine/MacBook\ Pro.sparesebundle
If your prefer to use quotes intead of the space escape, you should be able to do:
sudo tmutil inheritbackup "/Volumes/Time Machine/MacBook Pro.sparesebundle"
You will need to enter your Admin password, and that should do it.
I rebooted after this, then selected the backup share in Time Machine and now it's using the existing sparsebundle instead of creating a new one.