The car reads and retains the IDs of the first four sensors it finds after a cold start. I've had four sets of wheels on Hyundai vehicles and the TPMS works with all of them - as long as they are the correct sensors for the model and year. The only caution is that you don't want to have one set on the car and another compatible set in wheels in the trunk since you don't want the car picking up a mix of wheels that are in use and those in storage. Technically, the sensors are supposed to go to sleep when not moving, but I had my Elantra - rolling on tires with NO sensors (TPMS light would blink for a minute and then stay on solid) suddenly switch off the TPMS light during a drive. Reason - I had the OEM tires in the trunk - moving them from one storage area to another. So with the sensors in wheel/tire combos riding in the trunk, the car found them within about 10 minutes of drive time and the light went out on its own. Took those tires out of the car, drove another for another 10 minutes - light started flashing again - couldn't see any sensors.
Older MD Elantras use oddball Beru sensors so don't buy sets of 'work with most Hyundai and many others' generic 315MHz sensors and guess that they will work. If you say "learned" to mean flashed to be compatible with the correct year MD chassis, you should be able to install any set of tires with those sensors and have the car find them as you drive.
Perfect. I have a second set of 15" wheels that I plan to use and I have the correct TPMS's in them (52933-3X205). I was worried about having to go to a dealership and spend $80~$100 to have them "learned", but this post and one that you made last year indicate that isn't required. Sounds like I just need to put them on and drive for a bit. Awesome!
I've been swapping out 4 winter tires on TPMS-equipped rims with 4 all-season tires on the factory rims for the past 4 years with no problems whatsoever. As stated, so long as they're correct for the car, they'll be read correctly.