Yes, in case you have not used all your credits, they will roll over to the next month.
However, the amount of unused rolled-over credits can not exceed the amount of credits assigned to your monthly plan. For example, if the plan includes 5K credits, a maximum of 5K can rollover.
The unused credits are also carried over when you upgrade your plan at least 24 hours before your credits expire.
In case of a downgrade, unused credits will be rolled over as well. However, the amount of rolled-over credits won't be higher than the limit of your new plan. For example, if you downgrade to the plan with 1K credits, only 1K credits will be carried over.
In all the above cases, unused rolled-over credits are valid for one month only.
If you cancel your plan which sets you to the Free plan at the end of your billing period, all unused credits will expire.