It is probably just easier to sell or give away your current ownership and then find exactly what you need if anything to replace it. And when I say easier, I don't mean easy. Selling and even giving away weeks can be very difficult. You need to do some research to see what your week is really worth and you can't just go by the asking prices you see on Redweek. Some (if not many) are very unrealistic and will never sell for the prices listed. Very few timeshares are even worth 10% of what they were originally bought for. There are a few that might be worth 50% of retail but those timeshares are less than 1% of the total sold.
To do an exchange you would have to find someone who had exactly what you are looking for and happen to want exactly what you own. Nearly impossible.
Many find that once they get rid of their TS they are just as happy renting without owning so you might want to consider that option as well.
Tracey S.