Funny you mention this. I used a tool called Winclone (a Mac app designed for cloning Windows partitions and restoring them) to perform exactly this procedure (increase Boot Camp partition size without reinstalling Windows). You can't do it in Disk Utility, but it's really not that hard.
1. Convert your Windows partition to NTFS, if it isn't already
2. Clone the partition (with Winclone) to an external drive
3. Using Boot Camp Assistant, delete the old Windows partition
4. Relaunch it and create a new, larger partition
5. Cancel the install when prompted (choose Quit and Install Later)
6. Restore the clone you made earlier to the new, larger partition
Then find out which edition and license of Windows you have and perform a repair install. Huzzah.