Found:

Substitution salva veritate

SEMANTICS: the possibility of replacing an expression alpha by an expression beta with the same reference in such a way that the resulting sentence has the same truth-value. EXAMPLE: given that Beatrix is the eldest daughter of Juliana sentence (i)a is equivalent to (i)b in which the eldest daughter of Juliana has been substituted for Beatrix. Hence the substitution is salva veritate.

(i) a  Beatrix lives in The Hague
    b  The eldest daughter of Juliana lives in The Hague