Migration from old NAS to new

Pretty much.  If there are apps running on the old one, they will need to be removed before the migration since they are ARM versions and the new NAS is Intel based.  Then, re-install them on the new.  When you migrate the drives, you do them all at the same time with power off on both units.  That's probably what you meant, but I wanted to clarify.

There are some considerations as to whether he really wants to do that.  Age and size of the drives, and whether there is already backup or if the old NAS should be relegated to backup duty to the new are two main ones.  In many cases, moving to two or three of larger drives on the new is a better option, leaving an empty bay or two for future expansion.  And in the process, kicking down the road the point at which you are likely to have to deal with a failed drive.  But if cost is an issue, you can put that off.  At some future time, you could create the new volume of new drives on the old NAS, copy everything over, and then migrate again or migrate the volume back to the old one and make the new one on the new NAS then.