I think that this is a great idea pick for all the year and we can make changes afetr each slam or like you said every 2 or 3 months.
I like the idea to bring strategic to this game, maybe we can change 2 players before the "big" tournament of each Series but you will conservate the points of your original pick until that moment.
I also like the idea of Lisandro limitting the use of a player (5 per year)
1- Have the option to change players with surface change, after Miami, Roland Garros, etc.
2- Once a player is dropped, he can't be used again.