Is the premium plan a bunch of new features that weren't available in the beta? If so, I'd say just release them and send out an email.

If the premium plan is going to lock features that current users already have, I would put a delay on it going into effect for them... Send out an email saying they will be able to keep using the premium features for like a month or whatever, then they will lose them. Throw in a coupon code or another offer to help convince them to upgrade.