Paid credits will remain in your account if you have upgraded and downgraded plan. Free credits cannot be carried to a different plan. 

For example, if you are currently on a Pro Plan which comes with $5 free credits for a month and decided to upgrade to the Business plan which comes with $10 free credits. The $5 free credits will not be carried over to the upgraded plan.  You will only received $10 free credit from your business plan.

However, if you are have unused paid credit and you have downgraded to the Starter plan, these credit will remain in your account but you are not able to use it unless you are on the Pro Plan or Business Plan again.