The Shield of Samar was almost being completely renewed from inside out. The mech designers only intended to retain a few inconsequential parts of the current prime mech.
Naturally, they also planned to reshape much of the Unending alloy armor plating. The contours and physical dimensions of the prime mech and expert mech versions of the Shield of Samar differed substantially. New techniques, more refined methods and different physical requirements demanded a complete overhaul of the structure of the entire mech.
This presented Ves and the others with an interesting challenge on how to actually reform the mech.
In order to keep it alive, they had to keep the existing expert mech as intact as possible. On the other hand, if they wanted to upgrade it as comprehensively as possible, they had to swap out as much of its components.