Perhaps some numbers will illuminate this discussion and the magnitude of the diameter changes we're talking about:
Wheel diameter for stock 245/50R20 is 753 mm or 29.6".
Diameter for the 255/50R20 will be 763 mm or 30", which is 10 mm (0.4") wider, and 0.4" larger diameter, but will only increase your ride height by 0.2". Speedometer/odometer will be out by 1.3%, likely not enough to affect warranty. I don't know how much Chrysler allows for tire size variation, but Honda allowed up to 3% when I had my Odyssey. Lots of folks up here go up more than the OP's proposed 1% to increase winter ground clearance in the snow. Some Mitsubishis can go +5%, Jeeps a lot more.
Note that @BB21
's 255/45R20 are 738 mm or 29", 10 mm wider but SMALLER diameter, similar in diameter to the stock 17" and 18" setups.
Stock 17" is 235/65/17 = 29.0". Chains allowed: Max dia 29.7"
Stock 18" is 235/60/18 = 29.1". Chains allowed: Max dia 29.8"
As noted, interference with suspension components is a concern more than speedo calibration. For my snow tires, no chains allowed here so I went with 225/75/17, 30.3" and a bit narrower, which works better in the snow. No interference.
However, would this be counter-productive on a wheelchair van? Isn't it lowered on purpose? Would raising the ride height make the ramp steeper on deployment?
With respect to ride quality, tire quality makes a difference, but I will say that I noticed a clear improvement in ride quality switching from 18" to 17" when I got a deal on 17" take-offs. My dad has the 20" rims with even worse ride quality and he is going to 18's or 17's if he can find some cheap take-offs like I did.
One other note: When your tires are worn, higher profile 17" (smaller rim, taller rubber, same outer diameter) tires can be a lot cheaper to buy, pretty much paying for new 17" rims with the difference. That's what I found with my Mitsu - the 17" tires & rims weren't much more than new 18" tires & the $$ selling my old 18" rims was bonus.