To follow up on this, as it may come in handy for someone else...



I was using =CONCATENATE("="&F7&"+"&D7) to get started, and that works when the running total in F7 is just a number (the quantity used to date.) But when that cell is eventually replaced with the running "adding machine tape" that she wanted (=2+0+3+2), I ended up using

=CONCATENATE(FORMULATEXT(F7)&"+"&D7

Seems simple, but it took me a while to sort out what would work. I do that in a helper column, paste values over the F row, then use a little helper macro to refresh all those cells so it goes back to a working formula again.