I am new to AR pistol so please guide me. I ordered a PSA complete "multi cal"  pistol lower, and I intended to get 556 upper but now changed my mind and wanted to get a 9mm barrel. So I will need a complete barrel chambered in 9mm, 9mm mags, 9mm BCG? I beleive the lower has carbine buffer spring and buffer tube, do I need to change them? Your guidance is highly appreciated. Thank you.

Complete upper (includes BCG), mags, mag block to use those mags.

Carbine buffer may work fine, but you won't know until you try - and many go to an H1 or H2 to tame recoil even if it worked fine with the standard carbine buffer.

Mine needed a heavier buffer, 7.6oz works great. I bought it used and he had a PSA pistol buffer at about 4.5 oz and ten quarters. The heavier buffer runs like a sewing machine.

thanks guys... that's good to know

Heavier buffer is important in a blowback. Not just for function but for safety. The weight of that buffer is the only thing delaying that chamber unsealing and opening the gates of hell into your hands. I prefer most of that to have exited stage front before blowing back at me. A low buffer weight will at a minimum lead to increased debris coming back at the shooter, increased wear on the gun, and increased action noise.

This is good reading on blowback design.
http://www.orions-hammer.com/blowback/

May be easier to get it running if you just order a 9mm lower from PSA.
Either Glock or Colt.

I got the Glock mag version and it's cool.

If you go with a top loading mag block and a 5oz buffer (IMHO 5 oz is the lightest I'd go for a blowback) you can easily swap between 9mm & another caliber.

An H3 (5 oz) or 9mm (5.4oz) buffer should run fine in either setup.

LRBO in 9mm may be an issue. In my Hahn top loading block I have to run a long finger bolt catch which makes the block permanent. With a std. bolt catch it pops right out with the mag release button for quick swap. Bottom loaders don't have this problem but have to be set at just the right height so they're not 'quick swap'.

The Hahn & KAK blocks both have feed ramps which I think makes you setup more reliable.

I also run magwell grips on all my builds so a 5.56/multical lower & magwell block works better for me.

thanks for your input guys. I have decided to get a 5.56 for now and have already ordered a PSA 8.5" with a KAK flash can. I already have an H and H2 buffer ready for it.