Found:

demotion

MORPHOLOGY/SYNTAX: the phenomenon that a subject (or external argument) becomes an optional oblique phrase or adjunct due to some morphological or syntactic operation. EXAMPLE: if we form the passive of break (= broken), the subject (or external argument) is demoted to become an adjunct (= the by-phrase):

(i)  Tom broke the vase
(ii) The vase was broken (by Tom)