I don't have any advice, but I'll be watching this topic rather intently.
I had an idea (admittedly, a rather mundane idea) regarding and apartment-seeker website. The idea is on indefinite hold while I build something else, but one major problem I had with it was charging holding companies for ads before I had any visitors to the site (a classic chicken-or-egg problem if there ever was one).
The only thing I could think of was to begin charging from the start, but give the holding companies a sufficiently generous credit on the site to get them started - no risk to them, but it would give the site enough content to stand a chance at getting visitors to rent those listed apartments. The site gets bootstrapped, and the holding companies start paying as normal.
This approach might not work for you, but perhaps the same sort of "transition" could work. You approach your beta users with an offer of credit at your premium level for a set period - making it explicitly clear that they'll have to pay after that - then let them transition smoothly into the paid version. Perhaps you could even have some sort of segmented pricing where you offer a continued discounted rate based on how early they transition to paid status (i.e. for the first week, they can get three months at 75% off, second week is 50%, third week 25%, last week 10%, then full price therein). It puts a subtle pressure on them to sign up earlier, creating an artificial scarcity that might make them want in sooner.
This is all just off the top of my head, and I've never done anything like this before, so take this entire comment with a generous helping of salt.