Well, they've changed in the past... it's fine as long as the plans corresponding to the PlanIDs never get smaller or more expensive. This particular change violates that assumption and so is definitely not fine.
Presumably you can select the relevant ID based on plan properties (cores, RAM, etc.) rather than hardcoding IDs. Though I completely agree that once a plan has a specific ID, as long as that plan is available the ID should be constant.