Keep in mind that we are talking about use year, not calendar year.
For example, if you have an August use year, your 2008 points are good for stays between 8/1/2008 and 7/31/2009.
If you bank your 2008 use year points (which you cannot do before 8/1/2008), then they become valid for stays between 8/1/2009 and 7/31/2010. If you do not use them within that period, they expire and are lost to you.
Banking and borrowing are final transactions. They cannot be reversed. You may only bank a point once.