This plan makes no sense to me. The whole point of being a free app is that you get viral distribution:
1. Ten users use the app.
2. Ten users each tell ten friends about the neat free app.
3. A hundred users use the app.
4. Repeat over and over
5. ???
6. Profit!
Your proposed plan is more like this:
1. Ten users use the app.
2. Ten users each tell ten friends about the neat free app.
3. One hundred users arrive and find that the app, which they were told was free, actually costs $19.95 now. Eighty of them leave in disappointment. Ten of them sign up anyway. The remaining ten go back to their friends to ask what the deal is, find out about your grandfather clause, and put up angry blog posts about what an evil bait-and-switching moneygrubbing bastard you are. (See: Movable Type, Version 3.0, History of). Then they switch to your competition in a huff.
If you're going to have two tiers, make them different. Give people a choice. Don't try to openly tax people for not being early adopters, or for having last names that don't start with Z, or something. They won't appreciate it.