[Lava]

It all depends... There are mods that must be installed before you meet certain characters or enter areas where they are encountered. Sadly, mods that modify existing characters are that kind of mod that require a new game OR at least a save game from before you enter an area where they spawn.
Once a creature/character starts existing in your game, its current state is stored in your savegame. So even if you install a mod that changes that character, the change will only apply to your new game and savegames from before encountering that someone. The Viconia in your game won't be affected. Sorry.