I'm no AC mechanic, but I believe the core is welded in, so you'd have to take the old one off and weld in a new one. This wouldn't be a job to learn about welding, since the ever so tiniest flaw will cause a slow leak, and you'll be doing it again....
It's my (possibly ignorant) suspicion that a pretty good percentage of cores get replaced when they've really simply started leaking at a factory weld.... Of course when you're doing all that work you may as well replace it, but to have to do it just because the weld could have been done better in the first place....
Cape Canaveral, Florida
2005 Jeep Grand Cherokee Laredo 4.7L
2005 Dodge Durango SLT 4.7L