Agree with Candledave. The gimmick was easy to spot and the grid fill took little longer than a couple of this week’s Times puzzles. However I still have no idea what the theme is as I can’t find any interpretation of the title that fits the gimmick, let alone work out which letter to change or what the group is.
Next week almost certainly will be a numerical, which gives me a free weekend to work on this if I still haven’t got the theme!