Can I change it? Can I provide a virtual url that doesn't really exist but will take them to the right destination url?
When your ad is reviewed, one of the things the reviewer will look for is that the domain of the Display URL matches the domain of the Destination URL. So, as long as the domains are the same, you should be OK.
This is reviewed on a case by case basis by an actual human being, btw, not by an automated system.
AWA
why does the Destination URL and the Display URL have to be the same? Or is it jus the root domain?
Just the root domain. For example this would be OK:
Display URL: www.thissite.com
Destination URL: www.thissite.com?ref=gaw&id=12345
While this would not:
Display URL: www.thissite.com
Destination URL: www.someothersiteentirely.com?ref=gaw&id=12345
AWA