The $1500 is to upgrade the 18"s to 19"s when you order the car. That is not the same as buying a whole new set of 19" wheels. In your case, you will own two sets of wheels - the 18"s and the 19"s, so of course that will cost more.
Think of it this way - you could pay $2600 for 4 new 19" wheels and then sell your 18"s for $1100 and that would get you back to the $1500 upgrade price.
Tesla has not increased the price of anything - you are just doing an unfair comparison.