To make it easier for themselves, pretty often the destination URL is shown inside the ad under the title. So if you do change the destination URL after approval, it's very easy for them to tell it was changed; which is against their TOS.
For some reason though, the URL wouldn't always display under all ads. Never could figure out why.
The only time I've done it was when the offer was geo-ip targeted, so I would first have prosper redirecting to the non-aff link, and after approval, I would switch in my affiliate link. Wouldn't have any problems with this since the primary domain remained the same, so the display URL was the either way.