I decided to swap out the factory 20s on my 2016 Limited with a set of 17s off a 2018 SR5. I have my reasons. I couldn't find any definitive info out there before I swapped, so I decided to just go for it and see what happened. Hopefully this will help someone out in the future.
First thing I found was Toyota uses overlapping TPMS sensor numbers/frequencies. 2 of the wheels off the 2018 I swapped with would register on the tire shop's scanner as compatible with 2016, 2 did not. Therefore they sent me to the dealer to get the sensors reprogrammed.
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Second, the dealer cannot reprogram the TPMS sensors. They tried twice. Their efforts shut the TPMS light off for about 10 miles, then it came back on both times. Apparently you need to replace the sensors completely. At $150-$200 a piece (dealer quoted), I'll deal with the light. I have a couple of old cars I mess with so I'm used to checking tire pressures periodically anyway. Reprogramming may or may not work if you swap wheels with the same model year, but I cannot confirm either way.
Third, the Limited's spare is a full size 20" wheel and tire, just like the other 4 on the truck. The dealer could not confirm if the Limited has a 5th sensor in the spare or not. I feel it could, since it's a 5th premium wheel, but I can also see why it wouldn't since it's intended to be used as a spare. Additionally, don't forget to swap out your spares!! The 17" package is just slightly taller and a little wider. Mismatching tire height and width is not good for a full-time 4wd/AWD vehicle. The height difference might not be enough to cause any damage in the amount of time you'd be using a spare, but I'm not going to risk it.