While technically the answer is no, you can cancel your current upgrade and re-upgrade on the day of the month you would prefer to be billed. If you have already been billed for this month, please note that this charge will not be reversed and you would also be charged for the upcoming month as if you were a new client. None of your subscribers, lists or feeds will be lost and you can pick back up again normally after the upgrade is put back into place.
NOTE: If you want to switch from a monthly to an annual plan, the same process applies. Just pick the annual option from the form when you re-enter your billing details.