I expect it's a little animation smoothing and a few textures to bring the graphics up to date. Maybe one or two extra clothes and furniture they have lying around. Spa Day's yoga sessions could definitely use a little smoothing out by now.
Also, they'd bring it into line with Expansions like Seasons, Eco Lifestyle and Cats&Dogs, make sure the weather affects everything, all the items affect the environment, your animals can lie on all the furniture. Gardening, toddlers, water behaviour, Calendars, lot traits, all added to old worlds.
Maybe the NPCs will visit other worlds than their homes and their AI will be knocked up to date. Strays might wander around other worlds. If you own the right pack, NPCs will be using it, not just the pack they came in. It will definitely be an improvement, but not an addition.
Of course, Spa Day didn't come with a new world or proper NPCs, which makes it very easy to update, and its graphics are what, 5 years old, so we'll definitely see a difference. Mindless massagers might actually be turned into actual people, so of course you'll see a difference there too.
