Dear Charles, Dieudonné and Grothendieck themselves changed their terminology in the second edition of EGA I, published by Springer Verlag in 1971. At the end of their Avant-propos, on page 3, they write:

Signalons enfin, par rapport à la première édition, un changement important de terminologie: le mot schéma désigne maintenant ce qui était appelé "préschéma" dans la première édition, et les mots "schéma séparé" ce qui était appelé "schéma".

As to the suggestion "it was discovered that there were far more propositions about preschemes than about schemes, and people decided that this was ridiculous": considering the God-like status of Grothendieck and the awe he inspired, this sounds to me as plausible as courtesans  telling Louis XIV  "hey, this royalty business is pretty ridiculous. Why not name our country The Democratic Libertarian Republic of France?"