Best approach to backup and migrate Boot Camp
The best way to migrate a Boot Camp partition is to make an image of the partition and then restore that image to a blank partition.

Use Disk Utility to make an image of the BOOTCAMP partition, then repartition as 2 partitions1. Make sure that the BOOTCAMP partition is the same size or larger than before, then use the Restore tab to restore the image to the partition.

1 The repartitioning can be done with either Disk Utility or Boot Camp Assistant.