Please, do not add every possible tag just because they are available. You cannot have C-Mode and 7-Mode at the same time. I assume you have 7-Mode.
To expand aggregate you need free disks. Either you free some or buy some. There are no miracles.
When movin root volume all configuration is kept as long as no changes happens between copying of root volume and booting from new root. Make sure to schedule it with minimum delay. Theoretically it is possible that filer decides to change domain password at exactly the wrong time but that is easily fixed by resetting password.