Knoll Pollock Chair--casters?  

Kevi in Denmark
made that design first but others make it now. I believe this one (second photo) sold by Conveyer & Caster is the essentially the same thing, though you should check dimensions and, more importantly, the type of post.
(Also, the caption under the photo on C&C's page says "caster may differ from image displayed" but I think that just means that it comes in different metals, not just that one. But call and check. I ordered from them recently and talked to a very helpful and knowledgeable guy several times.)
http://www.cc-efi.com/Products/Casters/Series-14.aspx

ah, thanks! I will check...
ah, thanks! I will check that source....

The only casters I've seen...
The only casters I've seen on the older chairs were the chrome ball casters.
http://www.shepherdcasters.com/products_ball.html

Specs
Does anyone know the specs of the casters on the vintage pollock chairs? I'd like to swap them out for new shepherd chrome ball casters, but I don't know the size I need. I called both knoll and shepherd, and they couldn't help.

Maybe I'm confused, Josh.
Why can't you just pull one out and measure the post?
Or do you mean the ball diameter?

don't have them in front of me
I don't have one of the chairs at the moment. Considering purchasing a lot of them, but I'd like to know if I can swap the casters out.

I'm certain of it.
BTW, I did a quick image search and there are at least three different styles of casters on 4-arm bases. The wheels on Kevi twin wheel style casters are about 2 inches in diameter for reference.