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To increase flow, the hole is drilled out to match the spec on the BOM.
The threads on the shaft for the needle valve are chased with a tap to make sure they are nice and clean.
The shaft is inspected for any pits or cratches and if okay, polished to condition it for re use.
The BOM indicated that a different piston valve body is needed. This is probably done to increase overall rebound damping, particularly in the high speed area of the damping curve to make up for the increased needle valve flow. 
The new valve body is hand lapped on a surface plate to ensure it is perfectly flat. When dealing with low speed damping you are trying to get maximum control force with very low fluid flows so precision is critical. 
