The Startup and Enterprise plans both offer Credit Rollover where unused credits are automatically carried over to the next billing period.  

You cannot transfer rollover credits from the Startup or Enterprise accounts to the Free or Bootstrap plans so you will lose any credits that have accumulated in your account if you downgrade your account to one of those plans.

If you downgrade your account to the Free plan you will have 20,000 credits per billing period and if you downgrade to the Bootstrap plan you will have 100,000 credits per billing period to use.