Valves can be installed by individuals, though you should check the Internet to locate an appropriate goop. (You did make sure that the valves are proper for aluminum cylinders, right? Ones for steel tanks are shaped a bit differently, and it matters.) The most difficult part, in my experience, is removing the old valve because it's a bit challenging to keep the cylinder stationary.
Recertification requires the appropriate licensing in most states. (It goes without saying that your cylinders have to be in decent shape for this.) Get in the yellow pages and locate a dealer (not just a filler) that can handle it. Standard visual recertification wins you five more years of service. There's nothing special about aluminum cylinders for the recertification process.
Or hitch up, travel to New Mexico, and I'll get my dealer to recertify your cylinders.