Since the OP has not responded, I assumed the reason was to put the polarity sign in a more convenient place on the PCB, just as one rearranges names/labels to be readable. I concur that one needs to be careful. Eagle only allows limited choices for pins (attached). For polarized caps, the choice is passive, which means Eagle ignores the "+" sign. It is just like any other text. I suggested ERC, but I suspect that would not catch a cap labeled backward.
In any event, if it is just for convenience, I can see occasionally wanting to do that, but I would make it an alternative device of the same name. Nevertheless, there apparently is a way to do it.
Eagle may be woefully dated by current standards, but I have rarely if ever wanted to do something that I could not do with it. My version are 7.x.