You could add a new null. Hold shift while parenting the new null to the old null (this is new in Ae CS6) -- this will copy the transforms from the original null to the new null. (Pre-CS6, copy and paste PSR.)
Now unparent the new null, then parent the old null to the new null.
Your original null will now have zeroed-out transforms because they are measured locally relative to the new null, which had matching transforms in world space.
Walter Soyka
Principal & Designer at
Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events