LexArgent L&D is the same as any other pack you might add in regard to what it does with the existing sims and households. The only thing that does occasionally happen that might be considered a negative is that the game will turn a few sims who are used as walkbys through Mourningvale into ghosts. But that's easily fixed via one of the ways to resurrect a ghost. When it happens, the ghost loses nothing and remains an active member of their household (unlike when they would otherwise die in game). They don't get an urn/tombstone but keep their inventory, relationships, job, etc. so you don't lose any progress for those sims it happens to.
Hope this helps.
