Your 20" rims will come with lower-profile tires. That compensates for the difference in rim size. If there is any difference in total rim + tire diameter, the speedometer will be calibrated correctly.
If you look up the tow rating & axle ratings for the specific trim level of your truck, the 20" rims will be accounted for in that.
This assumes they were installed at the factory, and not some aftermarket item that the previous owner added. Again, the dealer should be able to run the VIN and find out if they were a factory option.