If it's like downhill brake control, everything is in place whereby you can add the switch and enable it. Hyundai won't fix it if it breaks though. You may have paid for the part, but you didn't pay for the support of that part.
Usually, at least lately, the trend is towards PnP in the auto software industry. If the device or devices exist, functionality is enabled. Now, if the device happens to be a switch, such as the Drive Mode switch, the ECU should understand on start that it exists, and if other parts of the system are in place to enable the functionality, it should work. It seems obvious, but I'm pointing out a philosophy.
It should automatically work if all the pieces are in place. Programmers tend not to disable software functions any more, they simply poll for devices and then configure for those devices.
Now, most manufacturers are not going to put the costly "pieces" on a lower trim model if they're not needed (AWD, duh), but they're certainly not going to run around disabling stuff because you didn't pay for it up front. They'll put what is needed, and if you want to add that other stuff later, well, that's on you.
BECAUSE: The cost to the manufacturer for part A vs. SUPER SPECIAL part B isn't but a difference of a few cents at most, in cases such as yours, where it's likely that everything is in place except the button. It's not like AWD, where there's a bunch of costly supporting hardware. You're not paying for that part when you pay for the higher trim levels when it comes to bells and whistles. You're paying for the SUPPORT of that part. If you get an Ultimate with that part (or package or system), then that part is supported. You paid for that support. If you add it later, even properly, with all the other appropriate parts (if needed), then those parts would still not be supported. The dealership would log the issue in for warranty work, and the manufacturer would reject it based on the vehicle's VIN. So, if it breaks, you fix it, and if it breaks something else, you fix it as well. If it all works, cool, it works.
Dealerships rarely make much off the purchase of a vehicle with regards to trim levels these days. That extra purchase price for a higher trim level goes partially towards hardware, in an AWD version for instance, and partially towards support of "technology packages" and the like. It's service, maintenance, and kickbacks from the factory, either through incentives or warranty work, that dealerships make their profit from. Manufactures don't want to support anything more than they have to, because the fewer warranty repairs the more profit. Conversely, the dealerships want just enough warranty and service work to make decent profits, but not so much to erode respect for the brand.
Anyway, blah blah blah. I never worked in a dealership, but I'm well acquainted with the industry, and that's why I'm running my mouth for far too long.
TL;DR: It will probably work, because drive modes are software-driven, but the dealership is under no obligation to fix it if it breaks, or it breaks something else, which it won't. It will either work or not. If not, it will probably be a well-handled fault somewhere, without any indication that anything is amiss... except your pretty button won't work, and you'll be pissed off that you scuffed such-and-such putting the darn thing in.
