Do rel canonical tags permanently affect link juice, or can they be undone later?

Does placing the rel=canonical tag pass the link juice to the canonicalised page forever? What I’m trying to say is, if one were to canonicalise page b to page a, would then reverting that change (for turning it into its own page lets say) leave some link juice and hence ranking "power" behind on page a or would b simply retain all that it had transferred via the canonicalisation signal it had sent to Google?