Hi @p8guitar. Yes it is only possible to edit main track / Section C chords. Chords from Sections A and B are considered part of the factory library i.e. included chord sets or a scale’s diatonic chords so editing those chords is not possible. Chords from those sections must first be moved to the main track before they can be edited.
There are plans to greatly improve the sketch page in a coming update and allowing the editing of chord notes there certainly makes sense, so thank you for the feedback and suggestions which will be taken into account. This is also a popular requested feature: Enable Righ Click Edit Chord on Chords In Sketch Progressions