TPMS light reset after downsizing tire and rim size

Before I posted this, I looked for about 30 minutes to try and find a similiar thread so as not to duplicate and I did find many. However, situations were a little different, and answers varied, so I am just going to throw out my exact scenario.
We purchased a 2016 CrewMax SR5 with the TSS Off Road Package last year from Texas. We drove it to Illinois which is where we live. We bought it specifically for towing our boat. After the 1000 mile ride home, we decided to remove the 20" LT off road Toyo tires and the TSS rims that Rockwall Toyota installed and we had Discount Tire put on 18" DX4 wheels and 18" Michelin All -Season tires. The sticker on the vehicle shows the air pressure on the 20's should be at 47 PSI. The recommended PSI on the Michelins is 35 PSI. Discount Tire took the OEM sensors off the 20" rims and put them on the 18" rims during the swap. We have been living with the TPMS light being on all the time in the dash and we would like to have that fixed after we remove the truck from winter storage for the boating season. Do we need to have the computer reset by Toyota to recognize a new and permanent PSI change? Will the reset button in the vehicle that I have been reading about (if our truck even has one) accomplish this task? Discount Tire was unable to use any of their equipment to fix the situation. One other related question: I did measure the diameter of the old tire/rim setup before we took them off and I measured the diameter of the new setup and the difference was only about 1/2". Do we need to worry about speedometer error with only 1/2" diameter difference? Your help will be greatly appreciated. Thank you.

 

I went from 20" to 18" and Les Schwab reset the sensors, no problems so far. As far as different tire diameter I don't get to concerned about it. I believe it is off by 3 or 4 percent.

 

There is a procedure for resetting the TPMS system. Going from memory I believe you have to have the ignition turned on or truck running with the transmission in park. Then there is a button on the lower part of the dash you press and hold for a few seconds. You’ll see dashes by the tire pressure readout if you’re on that screen. You then have to drive 20-30 miles and the system will reset itself to work with the pressure that is currently in your tires. I’m going from memory but I’m pretty sure I’ve got the broad strokes correct.