[KevinH]

I forgot the instructions ... In case anyone wants to try it:

unzip opf_id_updater.zip
cd opf_id_updater
python3 ./fix_opf_ids.py ./content.opf

It will spit out the updated opf to stdout.

Of course you can replace ./content.opf with the path to any opf you have lying around.
And you can redirect stdout to a file:

python3 ./fix_opf_ids.py PATH_TO_INPUT_OPF_FILE > PATH_TO_OUTPUT_OPF_FILE

I am sure there is an equivalent on Windows as well.