You could/should have installed the TPMS sensors from the 19" rims.
You only have to unseat the outer bead for access to remove the sensors.
They are mounted on the stem and easily come out, DO NOT MIX THEM UP.
Do not purchase another set of TPMS sensors and install them on the 18"s,
the ECU reads each sensor identification, so unless you go to the dealer, you can't reprogram it, AND if you reprogram the ECU for the new sensors, the original ones on the 19"s will not longer work unless reprogrammed again.
With that said, TPMS is far more important in the winter because of the contraction of air (drop in pressure) as the mercury drops. I would highly recommend that you have Nitrogen installed in the tires for winter driving with no TPMS. Hope this helps.