Your saved game files work perfectly fine after installing any new EP or SP, exactly the same as before. You'll simply notice the new Build/Buy items in their respective places ready to Build/Buy. Or you'll notice new interactions available to your Sims when you're playing them. It's a seamless integration process.
If the new EP offers new things to add to your town, when you start your game from the main menu it will bring up a box to allow you insert the new items. This is what happened when AMB was released and you got the option to add the new community lots to your saved games.
