[ownedbycats]

I use this template to convert uri: identifiers to gutenberg: ones.

program:
	publisher = $publisher;
	ids = $identifiers;
	u = select(ids, 'uri');
	g = select(ids, 'gutenberg');

	if 
		publisher == 'Project Gutenberg' 
		&& u != g 
	then
		n = re(u, '^http://www.gutenberg.org/(\d+)', '\1');
		ids = list_union(ids, strcat('gutenberg:', n), ',')
	fi;

	if 
		publisher == 'Project Gutenberg' 
		&& u 
		&& g 
	then
		ids = list_difference(ids, strcat('uri:', u), ',')

	fi;

	ids

To use this, get the Action Chains plugin and make a chain that applies a single-field template edit to the identifier field.



If you prefer different identifier, change 'gutenberg' in lines 5 and 12. You can also change the checks in line 8 and 16 to something like 'Gutenberg' inlist $tags.