Add those rtf files back to Calibre, then merge them with the original tagged .doc files to get all the metadata added without me having to redo it all?

That's what I'd do. Before you do that, open calibre, open preferences, go to Add books, check the box for "If books with similar titles and authors found, merge the new files automatically". That causes the new format to be added to the existing file for that book and preserves all of the metadata for that book.

If ever you need to merge manually, you can do that from the main screen under "edit metadata". You select the file you are merging into and then select the file to be merged (Ctrl+click) and then select the option from the "edit metadata" drop-down.