My primary concern deals with things I do not understand too well, so they make me nervous. (ABS, ESC, etc)
ABS and ESC, etc do rely on tire size as well as many other factors to decide how the systems will react.
Before these types of issues arose, it was mainly an issue of fit, speedo, and load-bearing capacity being high enough for ground clearance.
So, a small tire simply made the speedo 'fast', and you needed to check the weight rating on the tire to confirm it was still enough for the new car. Also, ground clearance and a "twitchy wheel" *might* become an issue.
I too in years past have changed both tires, and tires and wheels on many different cars with no issues.
All that said, on today's cars, I would not touch such a change with a 10FT pole! On my Tucson and Accent, when I bought winter wheels and tires, I made very sure that the "REVs" matched - so there would not be any issue.
I can not say *how* dangerous this might be - only that in my *mind* it *might* be very much so! So hence the "call the dealer ASAP" suggestion.