The MacBook Air is fairly easy to get into (I know, I had to do it recently.) It's just a few screws. I didn't change the hard-drive, but there are plenty of how-to videos on youtube showing how to replace the hard-drive.
The old ones, yes. But the newest ones don't have a 2.5" SSD in place: they use a bank of small chips instead (to help with the spacing), and they can't be replaced AFAIK: