Geman, H., El Karoui, N., Rochet, J.C. (1995) published paper "Changes of Numeraire, Changes of Probability Measures and Pricing of Options", on "Journal of Applied Probability " vol 32, pg 443-458.
This is a proof on the basic Numeraire changing methods in Financial Engineering.
May I check where could I find a copy?
You can find a copy of the article here.