Hi Martin,
There are probably a lot of different ways you could do this, but here are a couple of thoughts -
If you attach a formal requirement document to your Requirements, you can track the changes to the document as you upload updates. There is a checkbox on the upload screen to "Replace existing files, save original as a prior version" and the results look something like this:
http://screencast.com/t/ra5kXegBRIf you need to change the Requirement itself, not the attachment, you can always view the History of changes on the History tab.
If you like using Defects for gathering the information, you could add a Status for "Change Request" or a Custom Field for Defect Type = Change Request, Defect, etc., then use Traceability to link the Change Request type Defects to the original Requirement.