As I understand it, the corrector needs various additional modules such as a fiber-optically coupled diffraction camera for proper functioning. For best performance of the total system, the objective lens water cooling circuit is replaced by a new design that minimizes water turbulence. Further, the VG vacuum system is upgraded to an all-dry vacuum system comprising 2 ion pumps and a molecular drag pre-pumping station, for increased mechanical stability (no more boiling oil and evaporating nitrogen) and decreased sample contamination.
DUH!!
In other words, it's plumbed better. (It's always better to paint a visual picture).