Im currently running a Springfield armory saint chambered in 223/556 and want to shoot 7.62/39 can I just change the upper or is there more involved? Any suggestions on brands? Thanks

You can just swap the barrel and bolt (plus new mags) if you wanted to reuse the upper receiver.  But that would not allow for easy changes between calibers since you have to remove and replace the barrel each time. 

You could get a complete upper and new mags.  You might want to consider a stronger hammer spring for the lower receiver if you are using surplus russian ammo.  The primers are going to be harder. 

Any suggestions on complete uppers and magazines?

Here's other threads that's are on topic

https://2ahawaii.com/index.php?topic=27672.0

https://2ahawaii.com/index.php?topic=23134.msg207110#msg207110

Thank you both for your time and knowledge I appreciate all the help I can get. Thanks again.

No problem.

Here's another thread with some discussion of mags and buffer weight

https://2ahawaii.com/index.php?topic=25145.msg249730#msg249730

Any suggestions on complete uppers and magazines?

Have you made an upper before?  They are not difficult though need a few tools (torque wrench, vise and vise block to hold the upper).  That gives you a lot more flexibility to choose a barrel, upper, rail, bolt carrier, etc

If I was doing that swap I'd get an aero precision upper and then build the rest (I dont think Aero has a dedicated 7.62x39 upper)

Here is one of the uppers I made with no prior experience for a "bump-saw" build 

swapping out to a barreled upper in 7.62x39 may work as is, it might not
reliability and feeding ammo could be an issue.

I ran into some issues of feeding ammo and these are the mods I did to make it reliable:
7.62x39 bcg with enhanced firing pin (this came with the built upper)
wolf hammer spring (to make sure firing pin hits the steel cased ammo hard enough)
spikes T2 buffer (weight is 4.5 oz)
silicone/chrome buffer spring (7-10% stronger than milspec, and it lasts longer too)
widen the feed ramps with dremel
dremel the front lip of the 7.62x39 mags

now my AR 7.62x39 works very reliably   

My dad has a SOTA arms 762x39 upper and he needed to swap out the standard carbine buffer for an H3 buffer for it to function reliably. He bought some C Products 30 round mags but those couldn't feed anything more than maybe 17 rounds reliably, maybe those were duds or older production mags.
So I went to gunmagwarehouse and bought him some C Products 20 round mags and those have been flawless.

The knowledge I gathered and helpful suggestions has made me confident in moving forward. I will keep you all posted. Thank you again for all your help. I appreciate all the help I can get. Thanks again 😃👍

Good luck 
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