just my opinion.
if you are into bodybuilding, i think you need to eat at least 0.75 gram per pound of your bodyweight...
because protein help in building muscle and repairing the trained tissue..

if you are just into getting fit, i think you can try to maintain less than that amount.

i don't see any reason for you to replace protein. but if you don't wanna grow, then you can replace it.
you don't have to buy protein supplement if u are in a tight budget.
you can get them protein in food that you can eat everyday, and they are nice to eat also.
egg, beef meat, fish, chicken breast, etc etc..

please correct me if i'm wrong.

oh.. thanks for pointing that out