For example, a timeshare located on a beach would cost more to buy than a timeshare in Orlando. However, you can exchange your week in Orlando for a villa on a beach. I am calling this an upgrade. I have also heard of people trading for upgraded villas (for example, a studio for a 2 bedroom).
I think that one of the reasons Marriott has introduced their points (Marriott Destinations) program is to stop this from happening. Owners of the more expensive properties probably don't like hearing about people being able to easily exchange their less expensive week to stay at their more expensive timeshare property. With the points systems, this can't happen, because the points needed to get a beach property would be more than an owner of an Orlando property would have (unless they buy more point, and pay Marriott Vacation Club more money to vacation).
As of now, it is still pretty easy to use an exchange company like Interval International to trade weeks and obtain confirmations at awesome locations. As long as you do a few things right (like deposit a high demand week from your home resort)...see my post on Timeshare Trading Tips.
Don't be discouraged if you have been told at a Marriott Destinations sales presentation that you can't get highly desired properties through Interval International. It is possible, and has not changed, in my opinion. With the exception of Marriott Crystal Shores (see my post about this), I have had a lot of success using exchanges through Interval International.
