Personally? I would just replace armor with fantasy armor. Plate mail is replaced by turtle or ankheg shell armor. Maybe throw in some quests to get that shell armor.
Lot simpler, you don't have to futz with math. But I would also want to limit some of the spells, locks don't really make sense and many of the weapons.
I guess my point is that I don't see a reason to make anything more than cosmetic changes to armor if you aren't also going to redo all the other systems.
EDIT: ninja'd by
@DMMike it will also depend on level of technology you have.