I second promoting this as a sticky and suggest more do the same. This is extremely valuable for tons of noobs and experienced PT'ers alike.
@oldpunk and @POTSHOT Oldpunk, I took a Airgas CGA350 and migrated it to a CGA 320. My airgas store said a similar thing to me. It stems from FDA and CGA posters and regulations that say not to move used regulators from flammable or explosive gasses to anything any different. Doing so, may be dangerous. The salesman's point to me was, he couldn't do it. He said that sometimes gas gets trapped inside the regulator and when you remove the nut and nipple, flammable gasses can ignite and explode.
My personal opinion... I'll do it myself. I'm not hugely worried about this and I think you just have to use some common since. I look at this as just one more regulation/concern a big agency has to have to protect people from themselves. Just MY PERSONAL 2c.... Everybody else.... follow the rules. haha