What kind of upgrades can Apple Retail stores do on-site....

[JeDiBoYTJ]

im just curious what kind of upgrades Apple Retail Stores can do on-site to PowerBook G4s? Im sure they can install more RAM, install an airport card and replace the battery, but are they "authorized" to do other upgrades, like Hard Drive upgrades? or do they have to send away for that?

thanks in advance. :)