Change TFS Work Item Type
Not easily. You could try and hack the database, but that's really not a good idea. Just like dancing barefoot in a room full of broken glass whilst juggling live grenades isn't a good idea.
I'd suggest you create a new work item for the change request, link it to the bug and then set the bug state to obsolete/closed.
As noted you cannot directly change the work item type however there is a nice shortcut that may save you a lot of time for these scenarios. It will automatically copy links to change sets, test cases etc.
- Right-click on your workitem and select "Create Copy of Work Item"
- The following dialog window will pop up where you can then change the work item type for the new item:
Result: A new work item is created with the new type and the old one automatically added as a link and a comment added to the history area stating the work item ID it was copied from..