With Microvellum open, click "Microvellum Setup > Project Specification Groups."
Select the spec group component, to replace, in the left spec group window.
Select the new component in the right component window.
Click the "Assign Component..." button in the center between the two windows (See the workflow below).
Take note that the unused spec group components will display as orange text (all components using the same source file will display as blue text).
The use of the Assign button is similar to that used inside the material file for assigning pointers.