[JimMcIntyre]

I use a technique to teach the transition that seems to work...

Continue flying thumbs only but begin placing your forefinger to your thumb, kinda like making an "Ok" sign. Slowly (over many flights) transition the thumb backuntil you end in the pinch grip. Start with one hand and when you're comfortable with it, work on the other. You may find this easier if you lengthen your transmitter sticks slowly as well. You'll find it'll take more stick input to achieve the same maneauvers but you'll also notice this will result in finer control.

HTH. Jim.