Some existing screens already have the necessary technology inside, it just doesn't work because of incompatible Display Ports, those compatible display ports have now been standardised, so in theory with a firmware update those screens should be Free-Sync, its a theory, even if there is some truth to it they may only work in that way for videos, for gaming requires a little something extra of some description, apparently.
either way one might not know if one already has one of those screens, manufactures may not tell you, its also not in their interest to give you the necessary firmware update as they would rather you buy a new screen.