"I recently did some 'C' to assembler conversion for a Client: they weren't actualls changing the processor at the time, but doing a significant upgrade, and wanting the facility to later upgrade to a different processor"

"'C' to assembler" did you mean that ?

In that case we are talking about an approach I have seen used which is to have working and maintained code in C, which just happens to be a bit too slow. Then for each upgrade some functions are converted into assembler.
I do not believe (personal taste, not ironclad rule) this accomplishes anything, it is just as effective to maintain assembler for the speedoo functions as it is to convert C, even with a processor change.

Erik