Inertia matrix for modified body

[Aaron Henry]

Hello,
I am currently modifying a custom model which only utilized one side of a sheep pelvis. I have mirrored the geometry of the original pelvis to create a single full pelvis geometry file. My question involves the modification of the inertia matrix to account for the change in dynamics as the pelvis is now twice the original mass. From what I understand, geometry files are purely visual, so the inertial matrix should remain the same if my mass and center of mass location within the body frame are unchanged. However if I double the mass of the body (in this case account for the full pelvis rather than one half) my inertial matrix values should also double, assuming the center of mass location remains the same within the new body frame. Am I correct in my thinking? If not, how should I go about calculating the new inertial matrix?
Thanks,