Unfortunately at the moment you can only respell a chord symbol for an instrument with a given transposition. For example, you can respell a chord symbol on a B flat instrument by editing it and changing only the root: this will affect the spelling of the chord symbol for that and other B flat instruments, but will leave other instruments unaffected. But of course a guitar (while transposing by an octave) is essentially a non-transposing instrument, so changing the spelling there will change the spelling everywhere.
We do plan to make it easier to add chord symbols just for one player in future, but I’m afraid there’s no practical way to do this at the moment.