[necovek]

The best I can recommend is allow the free tier to move away their data locally (export): not sure if that makes sense in your context. That reduces the chances of them becoming pissed off (I always liked the distinction between British and US "pissed" though :)), even though it won't help with conversions much.

I don't think it's such a big deal anymore: users are already conditioned to expect entire free services to be gone tomorrow (somehow Google with Plus, Reader... comes to mind ;), so moving to being there but paid is not such a big deal.

Unless you promised it will be free forever (eg for beta testers etc).