I tried to do the opposite a couple of weeks ago for a friend. One of their party of 5 (currently on DONE4s in Oz/NZ) needs to come home and wouldn't be able to continue the RTW before the 12 months expires. I asked if she could convert her DONE4 to a Circle Pacific fare and AA said no, the two products aren't interchangeable. Thus she had to pay a reissue fee and return to the US (origin) the long way around. Big nuisance. They said (as it says in the rules) that you can move from a OWE to a GlobEx, but not to some other product.

I'll be curious to see if "up-selling" (going from a cheaper product to a more expensive one) earns the same reply. If it doesn't, good for you and it will earn them an angry phone call from yours truly.