If im doing different tunings I usually just make sure my guitar is perfectly in tune, then tune to the note I want on another string.

So if you want the G string an octave up for example, just tune to the 8th fret on the B above, or the 3rd fret on the high E. Just the same for any other string you want to retune, just find the octave above on another string.

Or you could just find the note on a tuner and do it one octave higher by ear, thats probably easier.