If you downgrade to a lower monthly plan, any credits that have rolled over from your previous plan will be carried over until your next monthly payment.
But you can only ever have 3x your current plans limit of credits stored up.
Bronze: You can store a maximum of 300 unused credits
Silver: You can store a maximum of 1500 unused credits
Gold: You can store a maximum of 4500 unused credits
So if, for example...
- you've signed up to the Silver Plan and have accrued 850 credits
- then you downgrade to the Bronze Plan
- when your next monthly payment kicks in you will carry over 300 of those credits.