hey how about keeping all your questions in one thread? yes they can be upgraded, most overseas airsoft sites sell upgrade parts, airsoftparts.ca is great and are located in canada.
aeg and gbb's use different style hopups, so their inner barrels are generally not interchangeable
Do some research on the gun you intend to buy and then see what other people have done to their guns, posting a thread for every random thought that enters your head is not going to make you any friends on this site.
read the faq and get av'd too