wish to change appleid on new iphone whilst keeping original id on old phone I now have two iphones.  both use same email address and apple id.My wife is to use the old one. I wish to have a new appleid on one of them whilst keeping the original on the other. how do I change the appleid on one without changing it also on the other. ? I am worried that when I change it on the new one it will also change it on the old one.
Unless you wish to spend the next year swapping back &amp; forth between IDs every time there's an update to something, best practise would be to treat it as though you were selling it &amp; wipe it completely, then set it up afresh with a new ID.

See What to do before selling or giving away your iPhone, iPad, or iPod touch

To get back any apps [though not their data] that were previously used on that phone, investigate Family Sharing which will allow any apps or music purchases to be shared between all 'family' members.