As a rule the answer to this would be no but had an idea and it seems to work.
Get the plugin BorderFx. This plugin work as both an export plugin ( the changes to the image are only seen in the exported image) or as an edit plugin (the changes to the image are stored back into the Aperture library).
While you don;t want to add any of the normal changes BorderFx would make to an image (borders, text on the image etc) the plugin does have the ability to save an image as either a TiFF or JPG.
So if you make no changes to the image and save it back as a JPG you should be able to do what you are looking to do. The only drawback is that you will have to do one image at at time.
Frank -- that does no conversion, does it? If it works, it works by creating a new Original. Though I suspose that could be combined with selecting the Image that has a TIFF Original and running "File➞Delete Original Image and all Versions".
This should only be attempted by those comfortable with Aperture's file handling.