Comment 2 for bug 1724589

One thought is that the ironic driver itself could update the allocations once a flavor is migrated and has a new custom resource class. That gets tricky for anyone that has already upgraded to pike and some of their instances / nodes have migrated - the nova code would have to check the existing allocations and see if they contain the custom resource class, and we don't want to do that check on every flavor migration run (which happens when nodes are refreshed).