1. Create a dmg backup of your Windows partition
2. Recreate the Bootcamp partition with the new size
3. Restore the dmg backup to the new bootcamp partition
I havent tested this, but in theory it looks fine. Keep in mind that when you create the Bootcamp partition, you will need to start the Windows install afterwards and let it copy the files, when it performs the reboot, then go ahead and do the restore of the dmg backup. This allows the creation of the MBR records that Windows needs to boot.
Also, make sure you have backups of your critical data as any modifications to hard drive partitions can pose a risk of data loss.