Saying the word upgrade is fine really, good in fact.
I don't think there's really a lot of people at this point that don't know about Wii U, maybe just really casual owners. The problem is that they don't really see a reason to upgrade yet.
I have a feeling this is definitely true. Still it probably doesn't hurt to do this now as well. I'm sure Nintendo will have proper tv ads when they have some bigger titles too.
Also another thing, the parents typically interested in buying a Wii or Wii U don't really care which they get (other than price and availability) if they don't have either one, they don't necessarily need the latest and the best in technology, the Wii is probably still relatively fresh to someone who never got round to owning one and is getting it for their young kids. Especially when Nintendo aren't pumping out the ads.
Good thing a lot bought a Wii.
I'm worried a lot of people will see the new adverts for mario 3d and mario kart wii U, see the flashy graphics and think - we already played that - aka misremembering how good the graphics were on the Wii. Nintendo are gonna have to do some heavy marketing but this applies to everyone really. Would probably be best if the next mario wasn't a galaxy game...and if the next mario kart lets you make your own levels