Apple ID question

[davidlv]

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.

 

[simonsi]

Are both AppleID's yours? why have two? If you do then yes you should be able to switch to the new one, depends on whether you use any Apps only connected on the old AppleID, if so then you may occasionally have to enter those details for updates etc.