If you've played around with extensions, you should definitely consider making a paid one. The APIs you need to verify payment are pretty interesting.

> Were the existing free users grandfathered in? It seems like it from the weekly users.

All user who had the extension installed when I made the switch were able to keep it for free. This was a little tricky for users who had installed, then uninstalled it. For those ones I just told them to email me for the developer version.

> Can you have two pricing models for the same extension?

You can't easily, but I did consider putting a lifetime version in the store as a separate extension.