You have until 2011 to use an exchange for your 2009 week. For example, if your 2009 deposit is for week 26, you can exchange for a week that you would use any time through week 26 of 2011. If you don't get your 2009 Spain exchange, you would have to expand your exchange request. The request won't automatically roll over to 2010.
Yes, as others have stated, you could deposit your 2010 week and request a 2009 week. However, as I tried to state above, your chances of getting the perfect exchange with that request are not the best, because you would be requesting an exchange less than a year in advance. Many deposits of the weeks you will want are deposited by their owners a year in advance and then taken immediately as exchanges. The most popular summer 2009 weeks will lkely be mostly gone (as exchanges) by the time you make your exchange request with your 2010 week.
Also, I don't think there is any advantage to requesting a 2009 exchange with your 2010 deposit compared to simply expanding your ongoing search with your 2009 week to include those other places for 2009 once it appears you might not get your desired 2009 Spain exchange.