I don't agree with the first piece of advice - I attach different templates to existing documents all the time. Depending on what documents contain (eg embedded metadata & custom XML), pasting content into a new document can leave behind vital components which might be required.
Yes, it is an age-old method of removing corruption to a document by copying all apart from the final return to a new document. However in my experience this doesn't actually do a great deal anyway. I think perhaps it used to work with particular types of document corruption but over the past 20 years, I have only seen truly corrupted documents a handful of times and in all of those cases the corruption was located elsewhere in a particular paragraph or in xml tags associated with equations.
The last pilcrow contains information about the page setup and header/footer for the final section in the document (and the final paragraph style and list format). As far as I know, it contains nothing else of note. So if the corruption appears to be in the page setup of the last section I would probably use this method as a solution.
There are have been other threads here in the last few days talking about corruption of lists caused by Sharepoint multi-authoring and/or editing via the browser. I would say those are more pressing issues than worrying about accumulation of garbage in documents.
20 years ago we used to see issues with proliferating list templates in documents (refer to this thread for instance (
Topic: Document Corruption @ AskWoody). I believe this issue doesn't persist as MS seem to have corrected the copy/paste of legacy list templates somewhere along the way.
So in short, I agree with your colleague - update the template and apply it to your existing documents. Actually, I would recommend you amend the existing template or rename the new template to the same name as the old template and that means the old documents get access to the new template without the user needing to do anything to 'attach' it again.