I've read a lot of posts on doing basic wheel size changes on the forums and it seems like it can be done without problems. I have a 2019 Lux with 21" and would like to go down to 20" for a better off road set up (actually 19" would be ideal but it seems not a good selection of AT tires in 19" out there). In my research I called the dealer to see what they'd say about finding 20" wheels and they said I could only go UP in wheel size on my Disco, and not down, because "it would mess up the suspension".

This sounds like dealer BS, but of course I need to find out if there is any truth to this. Does anyone know? And how about voiding the warranty? Thank you!

 

No issue with the suspension. That sounds made up...

You can downsize the wheel and use a tyre with a bit more sidewall to give you the same total rolling diameter. So it won't impact Speedo, cause ride height or clearance issues. etc

However the only consideration is whether you have 20inch wheels of the right specs (hole pattern, center bore, offset etc) - could use other LR ones. And whether the smaller wheel will clear the brakes.
20 should be fine and I think you can go to 19 for some wheel types but you would want to check brake caliper clearance

 

As BritCars said 20's should be fine its just some dealer BS my buddy downsized from a 21 to 20 and his disco is running fine mine is sitting on 20 as standard so I have no clue so you should be fine

 

Considering that you can order a Discovery with 20" wheels, sounds like the dealer is trying to upsell you.