I have been using Bell Hammer Seven at about 5% for several years in my Skyline GT-R BNR32 RB26DETT. If I switch to Bell Hammer Phantom, should I also add 5%, or should I use a full amount for the first application?

You can switch to Bell Hammer Phantom at the same 5% ratio during your next oil change. Since a protective film has already been formed from your previous use of Bell Hammer Seven, there is no need to use a full bottle or perform a special flushing process.
If you want a more noticeable change in response and film renewal, simply add Phantom at 5% to fresh oil during the next oil service.
Using Phantom at 5% is sufficient to obtain its performance.