Yes changing partition sizes with Disk Utility is very easy but Bootcamp won't boot afterwards! Read the many sob stories on Bootcamp forums.
There are two ways to resize a Bootcamp partition that I can personally vouch for (not saying others don't work, just that I have done these successfully).
1. Winclone. Make a Winclone backup image of your Bootcamp. Delete your Bootcamp partition with Bootcamp Assistant. Recreate your larger/smaller Bootcamp partition with Bootcamp Assistant. Restore the back up image with Winclone.
2. iPartition can resize partitions without messing up Bootcamp bootability.
PS I would recommend the Winclone route personally, since it is also a great way of backing up Bootcamp.