Let your early adopters earn free service by referring others.
Instead of alienating your biggest fans and supporters and power users, you'll be turning them into promoters and marketers.
As the other person said, if you offer a grandfather rate - you might have people who are signed up for life!
I think you need to study companies that transitioned from a free to a paid product rather than compare yourself against businesses that increase their regular sales through free samples.
So I'm not necessarily moving from a free product to a paid product. It's from a paid product that was subsidized, to a paid product that is not subsidized. But to your point, I think the closed beta ran too long and shifted their psychology about the cost of the product.
If it's all transparent, maybe that would make it easier for people to know that it's not free for free sake and plan accordingly.
I was in a similar position many years back and learned the hard way that people _absolutely loathe_ when you take things away from them.
Many of the testers do have free service (from referrals) that they'll get. But after that, they have to pay. I'm feeling pretty strict about it this time around.
If the marginal cost is zero (i.e. this is software) they are worth more to you than any kind of incremental fee (i.e. assuming a few dollars a month, not thousands).
It's just one of the most fundamental rules of business that increasing prices is one of the most disastrous things you can do where there are expectations.
It's why there are granfather contracts all over the place.
Make your new customers delighted with your product at 'some price' and your 'alpha/beta' people feel empowered that they get it for free. You can sell them addons later.
I'm not willing to spend the effort to build a custom billing system and pricing model just to grandfather these users in.
I think the mistake is to have unlimited messages if messages are such a major cost factor. Most services with "unlimited X" have a soft and hard limit on X if you dig deeper. If you don't want to charge per usage (or for overages), you should do a mixed calculation based on typical and min/max usage per customer and how usage ebbs and flows over time.
For the grandfather plan I'd aim lower than that even if it means losing a little money on high use customers because their usage might decline in the future (at which point they're providing profit while being less likely to cancel because of FOMO) and because the lower price point and legacy will make them good multipliers, not just for direct referrals but also as references or partners for whitepapers.
Basically you can get more out of them than just money so treating them the same as later customers means missing out on that.
I wasn’t, and don’t think other’s were advocating losing money on the originals.
Making them angry when they were on your side and helped you off the ground is a silly policy. The negative word of mouth factor alone could be incalculable and do unbounded harm, all for ~$100 bucks a month?
Treat them right and word of mouth will make much more than that in the long run.