Previous saves are compatible with FoV, and after installing it you will be able to load your latest save and keep playing, or you can create a new level 35 character and begin at the add-on content (you can do that in DKS, as well).
DKS contains too many changes to enemy placement and loot generation, etc, to maintain saved game compatibility with D2:ED.
There should be a patch released soon to convert D2:ED + FoV into DKS, so you could install FoV now and continue, and hopefully have a (very large) conversion patch available (which would break compatibility with previous saves) before you want to start a new character and play again from the beginning.
Welcome to the forum.
