Hi all 1st time being here. I have a question about my 458 socom
I have a multi caliber lower, is this only for the 458 , or can I change my upper and barrel for another caliber?
thank you

You just want to pop the top and pin another upper onto that lower?  No problem.  Even if it said 5.56 you could do that.  Caliber markings on a lower are just that, markings.  

What are you thinking of adding, 5.56, or something else?  As mentioned, check the buffer to see if it is compatible.

https://www.riflegearlab.com/what-calibers-ar15-lower/

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.

Tromix received approval from ATF 25 years ago to eliminate the caliber markings from the lower altogether.
The caliber should be on the barrel anyway, just like a bolt gun.
I'm surprised manufacturers are still putting the caliber/multi on lowers today.  It's been long overdue, getting rid of that practice.

Tony