Charts are always stored as Nashville Numbers

Every chart's underlying content is the Nashville Number System — chords are scale-degree numbers (1–7) that never change no matter what key you're viewing. That's the canonical format; it's not something you turn on or off.

Prefer to write or read in letter chords instead? Pick a Display Key up top any time — the chart reflows to that key instantly, and you can switch back to Nashville Numbers (or a different key) whenever you like, since nothing about the underlying content changes when you do.

Each chart just remembers what it was showing when you last saved it: save while looking at Nashville Numbers, and it reopens in Nashville Numbers. Save while looking at a letter key, and it reopens on the song's first Preferred Key instead (a song can have more than one — the first one wins as the default), not the literal letter you left it on, just a sensible starting point to write or read from.

Minor keys aren't in the Display Key list — pick the relative major instead (e.g. Am → C) for the same chords, one less mode to maintain.