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).
With the upcoming 'fabled' conversion patch, you would have the option to stick with D2:ED + FoV and maintain save compatibility, or install the patch to get the changes in DKS. There has not been a detailed or exhaustive list of those changes yet, or a lot of detailed feedback from people who have started playing DKS.
How different is the remastered edition?
