Can you switch to 20x1 1/8" or 20x1 3/8" wheels from 20x1.75" wheels?

20x 1 1/8" and 20 x 1 3/8" are 451 mm, but 20x 1.75 is 401 mm.

It's unlikely that you'll be able to switch from the bigger to the smaller if you use rim brakes. And it's not at all certain that the bigger will fit in a frame/fork made for the smaller.

I mean wheels. I saw that the 1 1/8, 1 3/8, 1.5, and 1.75 inches were the common wheel thickness. The bike I want comes with 1.75 inches stock, but I want 1 1/8 inches if I can.

it is going to depend on the width of the hub,

also the frame - diameter is 46mm smaller, geometry may be weird, BB hit ground ,etc

,

20 x 1 1/8

20 x 1 1/4

20 x 1 3/8 451 mm Juvenile lightweights, BMX for light riders, some recumbents

20 x 1.5-20 x 2.125 406 mm Most BMX, juvenile, folders, trailers, some recumbents

tires woudl be different

You mean tires or wheels? Generally you don't mix fractional with decimal (inched) sizing.