Product change on the fly

Hi,

I was wondering what will be the right way to change the product attach to the content type on the fly.

Scenario:

I have a content type call registration and two products one call class and the other event. The registration content type reference another content type where I define the type of registration that it will be, so I can use the "hook_form_alter" to check for it. Once I know the type of registration I should be able to alter the form to insert the "nid" of the right product so that the right product attributes are loaded.

Note: I know I could create another registration content type, but I am trying to keep things simple and make it transparent to the site manager.

Thank you.