Fred:
I may have misread your question. Do you mean when was the tender "upgraded" and the large airtank added?
I think the headlight bracket was replaced in it's forward position to allow the fake oil-style box headlight to clear a fake diamond stack for the 1949 exposition. That was part of the "Montezuma" disguise. I don't know about the extra large air tank. That seems to have come later, in the 50's, but the tender was modernized bi 6/49. The pilot flanger was added in the fifties, too.
Mike