Isn't the softphone part of the user account and a physical phone a separate account? I'm trying to grasp what you are asking for - you are wanting to reassign a softphone, which is a user to another user? If so, (not ideally) but you can always rename the users account (but beware all the original users configurations carry over).
A softphone is not really part of the user account, despite how tightly RingCentral integrates the two. For example, if you delete a user, the softphone will become unassigned with its associated phone number. I have about ten unassigned softphones that I cannot rename. Of course, I can always rename the softphone before I delete a user, but my point is that I shouldn't have to. You can easily rename a desk phone, so why can't you rename a softphone? It's a basic feature that's lacking.