if for some reason a choir cannot sing one or another liturgical text according to the music printed in the liturgical books, the only permissible substitution is this: that it be sung either recto tono, i.e., on a straight tone, or set to one of the psalm tones. - De Musica Sacra (1958), 21. c)
This doesn't exclude polyphony (ibid., 17-18).
That does seem to be the logic of taking the instructions at face value.
By the way, the setting in the older book might or might not really be older than its replacement: it might take some digging to determine that.
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