Yes, if you have a monthly subscription, unused credits will roll over and accumulate in your account up to 2x your monthly credit limit.
For example, if your plan includes 2,000 credits per month and you have 500 credits left at the end of the month, you'll start the new month with 2,500 credits (2,000 from renewal + 500 rolled over).
If you downgrade to a free plan, you can only rollover up to 100 credits.