Actually, the change would need to be from Manual to Auto, and I would
imagine that a change in Page Setup alone would be enough to mark the
template as dirty, so the "phantom edit" would not be required.
Moreover, it should not be necessary to even open the template to do this.
In a new document based on the template, without making any other changes,
change the paper source in Page Setup, then click Default..., and answer yes
when you're asked if you want this to be the default for all new documents
based on this template.