I wouldn't restart, and I don't really think it is a good idea:
1- no real reason, 0.21f is quite stable. A game like this lasts at least one year if we don't give up, are we going to restart at every new minor version ?
2- 0.22a is unstable and not yet bug free
3- there aren't significant changes, and actually I think those new changes are in need heavy balance tuning for now.