Will I be charged for the entire month if I cancel my PRO plan subscription?
No, you will not be charged for the entire month. Your card will be charged on a pro rata basis for only the number of days your team was on the PRO plan.
E.g. If the current billing cycle for your team is May 26 to June 25, and you decide to cancel your subscription on June 1, you will be charged for all active users on a pro rata basis from May 26 to June 1.