There is an old joke; how do you make a guitarist stop playing? Put sheet music in front of them!
Let see if I can help you a little bit in this regard by helping you to convert a major key to it's relative minor key.
In big the big band in college our leader would often change the key of songs on the fly. I had to learn the relative pitches quickly and this is my cheat method for transposing major to minor and vise versa. It's easy when you have time but this still can be a challenge when you have the whole chart in front of you and the change is instantaneous.
*This simple trick could come in handy if you know all the notes on the neck already because this works on all strings.
Determine a root key. Take the key of A for example: Fret the note A on your 6th string (sometimes called low string). This note would be fretted on the 6th string at the 5th fret (either your high or low E string will work). Now slide or count down 4 frets. The note fretted will be F# (C-sharp). Therefore, the relative minor of A Major is F# Sharp.
Try another: Fret the 7th fret of the 6th string = C, move down 4 frets and you are now fretting A. Again, Fret the 9th fret of the 6th string = C# major, now slide or move down 4 frets. You are now fretting A# minor. Keep in mind that there is something called enharmonic keys where notes or keys such as C# could also be called Db but they have the same pitch and use the same key.
Learning simple theory can help you trick out your playing without having to do finger gymnastics.