If you have a quality .458 SOCOM upper then you are using a standard buffer and spring anyway, either rifle or CAR, and the blending on the feed ramp is done on the upper, not the lower. So, if you have an any caliber upper that you want to swap the .458 SOCOM upper with, have at it. The .458S was designed from the ground up to be a field changeable upper with the 5.56 without the need to change mags, buffers, or springs. All you do is pop on the upper, insert the magazine with the appropriate cartridges loaded into it, and rock and roll.
Legally, as already said, caliber markings on the lower mean nothing.