I would say yes, and definitely yes.
Or, you can let the early adopters use the site for a while (6 months?) after you convert. Remind them monthly that their special treatment will end on a certain date, so they are not surprised.
Also, how much does it cost you to let them use your service. Often it is zero. If so, why not let them keep using it for free?
It's also a good idea to have a free option MVS (Minimum Viable Service), regardless of how many people use the service before you start charging.
Don't overestimate how many users you will have, even if you give it away for free. You may be surprised at how difficult is to get people to use your service, even if it is free. (I am speaking from first-hand experience.)
I suspect that given the small number of users and low cost it will make sense to keep them free. Once you are big, rich, and successful, you can tell them they have to pay. Until then, having more users will generate momentum.
Another idea is to ask a favor of them. "We are converting to a paid service. Since you have been an early adopter, we are going to allow you to continue using the service for free. Could you do me a favor? Can you give me feedback about how we can improve? What made you decide to start using the service in the first place?"