Thanks Peter.

Indeed, it's certainly helpful to know what's going on with NA's.

In your example, Gm7-C7-Fmaj7 works perfectly well over the original changes.

But if the lead sheet says Calt and the backing harmony has become C13, the soloist is going to wonder why things sound a bit odd.

But no problem: next time Herbie drops by to sit in, I'll have a word with him...

Best to All - Andy