2021 Crf450 Wheels

90% certain you'd be fine for the wheel swap. Front Honda hubs haven't changed since about 95', and the rears fit multiple models for extended periods (CRF450 from 2002-2012, 2013-Current). Checking the info from the new model specs, it appears to be the same rear with 240mm rotor and spacing; just can't be 100% till I actually see one. See pic for 2021 look. Personally, I'd wait on the rear for verification, but that's just me.

Interesting - Honda dropped the rear disc guard in 2020 (for weight, less heating, whatever), but left the two mounting points on the rear caliper hanger. Going out on a limb, I'd have to bet they had TONS lying around and weren't about to retool. Only noticed this now from the photo, but it's a pet-peeve when seeing proper bike builds leave them on...

I'd be careful with buying them ahead of time and trying to judge fitment by pictures. Remember the debacle of the 2012-2013 rear wheel change. The only difference from 12-13 was the hub moving the sprocket mounting surface 3mm outboard. Otherwise, the hub looked exactly the same and the wheels would "bolt up" between the bikes no problem. Honda didn't really announce this change and the way people figured it out was when they put their 2012 fancy wheels on their 2013 and got launched over the handle bars when the 3mm difference caused their chain to derail.

Perhaps a company of the size of Dubya has some early info on the '21s Wheels? A call to them might give you an answer.

2021 Update - Apparently the rear is new. Multiple media outlets addressed this in the intro videos, so best to wait on the new wheelset if you can motorscross!

Worse case you can buy different end caps. Same was the case with the 19+ Kx450.

I thought the OEM pre-2019 front hubs were not big enough for the bigger axle on the KX450?

Not even close to what the worst case could possibly be, LOL.

When Honda did the 2012-2013 rear wheel change, as I posted above, they moved the sprocket mounting surface 3mm outwards. I'd say getting launched over the handlebars is a little worse than buying end caps, LOL. Once people figured this out, the options were to buy a 3mm sprocket mounting spacer plate or a sprocket that had 3mm of offset built into it, assuming you were still riding after being launched over the bars, that is.