Please, instead of trying to rebuild the command string, just use _execvp instead of system. (_spawnvp would also be ok, but the difference doesn't especially matter in this case.)
You can already do that: just reduce the amount of oxidizer in your fuel tank to zero. The only thing that switching fuel types would give you would be the ability to put more of a given fuel type in a single tank. Also frankly, removing redundant parts would be pretty nice!
Believe it or not, Squad has posted an approximate release date, although I don't see it on the official forums. I can only assume that they have chosen to mention this off-site because of bad experiences with the community here. Or they just forgot.