While you can bank 2003 into 2004 and then later borrow 2004 to use in 2003, keep in mind that you are limiting your 2004 options.
If you bank 2003 points into 2004, you MUST use those banked points in 2004 or lose them. You cannot bank them again into 2005. So if you do bank your 2003 points, make sure you have plans to definitely use them for sure during 2004.
Also, you don't have to make a reservation by July. You only have to decide whether you want to bank points or not. If you know for sure you'll be taking a trip yet in your 2003 use year (Feb1, 2003 thru Jan 31, 2004) you don't need to bank at all. Then in Aug/Sept when you've settled on dates, make your reservation using 2003 points. If it doesn't require all your points, you still have until Oct 31 to bank points (up to 50% of your total)
Example, you have 150 points, don't bank any by July 31 because you know for sure you want to go sometime in Nov/Dec. In Sept you make a Nov ressie using 80 points. This leaves 70 points. Since 70 is less than 1/2 of your 150 points total, you still have until Oct 31 to bank those 70 points.
If on the other hand, come July 31 and you are not firm yet that you will definitely be going sometime prior to Jan 31, 2004, then you would need to bank all your 2003 points, and then as stated borrow 2004 points later if you change your mind.