Converting ordinary posts to "product" custom post type :(?

Please help ... this could be a life saver.

Here's the thing: I offer help on hundreds of thousands of academic questions (~ over 200,000)... each tutorial could be customised as to how much help the student wants with it... like x dollars per page, and rate goes up if its prioritized, and some specifications like the frmatting of the tutorial etc.

AT PRESENT: All these questions are in form of posts - ordinary WP posts.

I think that Marketplace plugin uses custom post types to make a product out of a post.

THUS THE PROBLEM WOULD BE:
Im wondering how can I convert all those posts to the custom post type of "products" so every one automatically is up for selling. AND - I could make product variations etc for each question, so the problem would be resolved.

TWIST:
The twist to the tale is, when kids post new questions - they are set to appear as normal posts on my website. Would I have to intervene everytime to convert them to products..., if we could make a permanent edit to post.php may be it could help? Or do something else?

My website is (a s s i g n m e n t h o l e dot c o m) - please remove the spacing!

In anticipation THANKS a tonne ... and I need this like medicine, PLEASEEE help!!!