You probably should have thought about this issue before you started the beta, but now that the moment is here... You could, of course, do the crude "Your subscription will expire in X days unless you buy a subscription,"but that might create ill will in your users since it's been free all this time, and people are very used to everything being free.
I saw a little bit of a similar issue with Xobni. They had their product free for a long time and added a lot of features over time. They recently went to a freemium model, but they took some features that they already had and put it into the for-pay version of Xobni. I have seen plenty of angry users because what used to be free now they have to pay for. It's not a rebellion of Xobni's users, but it's enough of them that I would count it as a mistake on Xobni's part.
Anyway, I think you need to give your users an incentive to switch to the paid account. When you go live, you need to also deliver new features that will compel people to switch to a for-pay account. You need to figure out what to do with your current free users, if you will allow them to continue to be free, or maybe give them a discount on switchover to a paid account.
One thing you probably cannot expect them to do is to go in droves to paid accounts without you giving them anything in return. Discounts, new features, or whatever it is, you need to make those users feel like they are gaining something that was not there before so they will actually want to sign up and pay you, otherwise you will repeat Xobni's mistake and your users will feel like you have their data hostage because now you will force them to pay for something that used to be free.
EDIT: I will clarify that when Xobni created a for-pay version they did add a lot of features, so it was a major upgrade at the same time. Some of the features that used to be free, however, became for pay. This is what irked some users who would have switched anyway, myself included.
Also what made it bad was the sneaky nature of the switch: People who already had Xobni installed were allowed to keep the features, while new installs popped up an annoying "Pro" icon that basically says ïf you want this, pay". I noticed it because I reinstalled my computer and now I could not do what I did before. Anyway, nobody is entitled to "free everything" (And that's why I am a paying customer in the end), but I think they could have handled all of this a lot better.