So, on a couple of my guitars I have this issue where each string doesn't stay in tune when I move up the neck... like if I were to play an open E, then play the 12th fret on the same string, the octave is out of tune with the open string. I thought it was maybe the strings themselves were just old (they were), but I changed the strings recently... and I'm still having the same issue. Is my neck warped or something like that?
yes