answer:As far as I know, there is no legal way to do it. I also have to wonder why you have so much R-12, but I’m not sure I really want to know. Now, if you manage to find replacements for all of the parts that were replaced during the retrofit, you might be able to undo the harm and run R12 again, but I’m doubtful that you can find components made specifically for a refrigerant that hasn’t been made for 15 years. Personally, most of the people I’ve heard kvetch about retrofitted systems also hate fuel injection, solid-state ignition, front wheel drive, and in some cases indoor plumbing. Any system old enough to have had R12 in it before everyone went to R134 is probably rotted to the point where it won’t hold much refrigerant of any type anyways. True, R134 has a smaller molecule and will leak through hoses designed for R12, but at this point, so will R12. My recommendation is to sell that R-12 to some garage. A case of twelve 12oz cans goes for over $300 easily while a 30-pound can seems to be around $600–700 right now. I think that you might be able to get enough from the sale to get the system redone right.