Can my bolt and carrier work in any upper that is mil. spec.? I have a colt sporter II pre ban with A2 handle. I would like to have a flat top but dont want to buy a whole new one. So my question is will all the parts from my old upper work with my new? Thanks in advance.

As your replacing all the old parts (barrel, FSB, delta ring, snap-ring etc...) into a "new" A3 upper receiver, YES this is okay to do.

FWIW, while one can use an old b/bc in a complete new upper (note: new receiver/barrel/barrel extension, etc...) and it would probably work, this would NOT be something I would recommend, however if I'm reading your post correctly, you should be okay. As your old b/bc has already been running with the old barrel and barrel extension, that'll be on the new upper receiver.

Swapping bolts between different uppers CAN be done, but I DO NOT do this, not in my rifles. Each has it's own dedicated b/bc/ch.

Mike