So you're rotating the UVs? not the mesh?
Due to the way tangent space normal maps work, you cannot just rotate them.
You'd need to do some channel swapping and inverting for 90° 180° and 270° rotations see these photoshop actions
. from this thread
For anything other than that you'd need to recalculate the normal information with something like this tool
made by Ryan Clark
from this thread
Hmm... looks like the link to the photoshop actions is broken, they used to be in the wiki as well but I can't seem to locate them, at least the relevant info is still in the post. Sorry...
of the wiki may help explain it a bit better.