Yes, if a Guru says we must redownload - like they did after the pets update - then everything had to be redownloaded from our purchase history for those updates to take affect - but generally it is just worlds and venues. But as you said the guru's genrally let us know.
But for our own items like lots we have made, the lots in game will not update by themselves, so as long as we have saved them to our studio, we can be sure to have our own lots updated by redownloading them from our studios if there were actual coding changes made in the game - and there was some in Seasons so certain items were weatherized. I know some roofs were also affected, but by uninstalling the ones I had in my library and redownloading the same one from my studio, the roofs were fixed without me having to do anything to them. But a regular update with just added coding will automatically go into your whole game when you install the update and ep it came with and generally you don't have to do anything.
The only things that will not get updated will be third party content not made by EA.
