Just bought an iPhone and switching to that agent's ISP in a week. My old Mac Pro has an Apple ID associated with it, and the iPhone has a different new Apple ID. When the ISP gets switched, can I just change the old Apple ID to the new one for the iPhone, or will I gave to do a complete fresh OS X install? Currently running 10.10.5 but I have an internal HD with El Cap on it, which mostly works fine too, I just prefer Yosemite. Any advice appreciated.