Pro is now $20 a month for your whole organization, not $20 per member. Invite as many people as you like, and your bill stays the same. Each month includes $20 of hosting credit, and hosting usage beyond that is billed as before.
If you're already on Pro, there's nothing to do. We'll move your subscription to the flat price before your next invoice. If you were paying for several members, your bill drops to $20 a month. The hosting credit you've already been given for this billing period stays as it is, and from your next period it's $20 a month.
See pricing for what each plan includes.