The romance skill and what this EP brings, in general, is more interactions to express how your sims feel about one another. So there will be more interactions depending on level of love skill and relationship dynamic. Everything in this EP is in addition to what is already there, so your existing couples won't be limited (more than they already are) in the romance department but they will be able to learn and unlock more interactions. I wouldn't say relationships are fundamentally different or more complicated, now there are the relationship dynamics, but this works basically like lifestyles and once you match a relationship pattern sims behavior will continue in that pattern and will open some specific interactions. If you want to change that you need to work on it. I like the custom relationships you can now add, though not sure those are tied to the EP. A sim now has the status of stalking ex-fiancee in my game.