The documentation for configuring the submission forms in DSpace 7.x can be found here:
https://wiki.lyrasis.org/display/DSDOC7x/Submission+User+Interface
If I read them correctly, the submission system does not work the way you want it to. You can define multiple different submission forms, but they can't be triggered based on the selected value of a dropdown. Instead, each form is linked to a specific collection. So if you had, for example, a collection to hold "Exam Papers" or "Dissertations and Theses", then when a person chose to submit to one of those collections, they would be presented with the appropriate submission form. This is exactly what we do. We have one default submission form for all DSpace submissions, and one specific to our Thesis collection, so when people submit a thesis they get fields customized for ETDs (degree name, program, etc).
It appears DSpace 7 also introduced the concept of Configurable Entities -- see:
https://wiki.lyrasis.org/display/DSDOC7x/Configurable+Entities -- which sound more like what you might be looking for. I have absolutely no experience with enties though, so it's possible they won't help you achieve what you want.
- Darryl