Cannot use existng Time Machine Backup on OS X Server after changing password
For me, I had to delete the old password from the login / system key chain before it would let me connect again to my server.

Also, Time Machine won't show you the old backups until after it makes the first new backup, so you might need to clear the old password and then set up the destination again and then wait for the backup to finish.

At that point, the interface should show the old range. If it doesn't inherit the old backups, you have options, but there are some steps needed to get there first possibly.