Lazlo,
I can confirm the instructions above work and I will bear them in mind for any future repeat projects.
Yesterday I ended up MERGEing the old project into a new file which went smoothly and forced me to update a few things which may develop the file's resilience for future updates. It is an option I have never tried so I was pleased with the result.
Thought I would post what I did in case someone else has a similar predicament.
As ever many thanks for your contribution!
Apple iMac Intel i9 / macOS Sonoma / AC27UKI (most recent builds.. if they work)