Any reason not to do this?

So, during your first ever differential changeover from open carrier to a rebuilt suregrip unit, you already knew everything and understood any issues that may come your way? Assuming this reply was directed at me and not the builder of the third member. Hard to tell here.

That is directed to the builder of the third member, not you. Your builder is no professional in my books and you should not just walk away, instead you should run away.