It depends on what the service costs to run. I really don't like a model where one customer pays for the other but promoting a new service is getting more expen

It depends on what the service costs to run. I really don't like a model where one customer pays for the other but promoting a new service is getting more expensive every year. It might be worth the insanity of large numbers of low tier customers. One thing is for sure, no one likes a service that changes prices with free > paid being the least likable. (paid > free as the nr 2?) I think you can get away by describing the situation properly in advance. i.e. what might happen with the free service in the future. Something like: Guaranteed to be free for 6 months (12 months if you register now). If we fail to reach an audience by offering a free plan it will be x dollar per month/year/query. We will contact you 3 months in advance. It seems the free pony! era has to end. Downtime from cleaning up after a vanishing free service is to annoying.