When I did something similar, I implemented it as a “virtual” model and applied validations to make sure that the PlanChange was properly constructed, and then performed the change in a transaction. I was able to test the hell out of it both with and without actual database interaction. But I was able to use it in three different locations and removed lots of special case code that had built up over two years of development.
You probably don't need Commands. You probably need something to orchestrate manipulating multiple ActiveRecord objects simultaneously in a transaction. It's still a model, but it may not be the model that you thought it was.
 Account from Plan [a] to Plan [b] that has a different quota, affecting Device assignments, possibly including moving some devices to Plan [b], some devices to Plan [z] (a default plan), and disabling some devices entirely because there's no quota available anymore.
Command, Virtual Model, Verb Model, Interaction. If I were working on your team I am sure we could agree that one of those would do for a name.