The answer is really very complex. Others have raised good points. Is your next home going to be one of the same value or more? If it's more, that's a case for paying extra. On the other hand, if you want to buy the next one before this is sold, you will need that additional money more as cash than as equity. Then there are interest rates. If you can safely invest and receive higher interest than your mortgage, that is a case for not paying extra. My own feelings are always to pay down your mortgage when you can. There's a feeling of safety and ease to this that I personally like very much. Making half payments but twice as often will help do this as well as simply adding extra to the principal. It can create some record-keeping nightmares, so make sure you are careful in your notes about how much and when you pay unscheduled payments.
If you were not planning on moving so soon, I would be stronger in urging you to make the extra payments.