Hey @anamorph,
Sure.
Put all your files in a directory.
Open the Terminal.
cd "<path-to-directory>`
To move the working directory to the correct place.
ls
To make certain you are in the correct place.
textutil -format txt -convert rtf -encoding UTF-8 -font Times -fontsize 18 *
To convert the files. (Change font family and size to suit.)
The originals are left intact, so you're okay if you make a mistake.
-Chris