1) When u convert from NURBS to Poly, use "Control Points"

this lets poly vertices to be where NURBS CV's are. (the hulls in NURBS will be the Poly edges aye?)

2) When converting Poly to Sub-D, turn "Keep original" OFF.

so there is no copy of it.

3) When converting back form Sub-D to Poly, set method to "Vertices".

that lets the Vertices of Poly to be exactly the way it was before.

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i think what you did was,

change it to Sub-D, with "keep original on".. then changed to Poly not using "Vertices"...

thus, having insane mush of poly vertices.
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i hope this helps