I just upgraded the drive in my 13" unibody MB and moved and enlarged my Windows Vista partition with
winclone.
I cloned the partition off to a DMG on an external drive, created a new partiton on the new drive and restored the image without a fuss.
So what you will need is a writable (i.e. NOT NTFS formatted) external USB drive that is larger than your current Windows partition.
Use winclone to clone the partition to the external drive
Delete the bootcamp partition in Disk Utility
Shrink the OS X partition
Create a new, larger bootcamp partition in Disk Utility out of the unformatted space, format it FAT32
Use winclone to restore your partition
Boot the restored partiton
re-activate windows (if necessary)
EDIT: If you are uncomfortable with Disk Utility, use Boot Camp Assistant to restore the drive to one partition and create the new partition, while still using winclone to backup and restore.
NOTE: It's always good to have a full and up to date back up before attempting any of these kinds of operations.
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