The General Instruction of the Liturgy of the Hours (246-252) discusses substituting texts "in particular cases" (246), but not on Sundays or feasts (247). Substituting one Scriptural reading for another is discouraged (248), but not mentioning other parts of the office implies it's more acceptable to change them. The wording throughout this section is generally weak; compare it to 282 which says the responsories "demand to be sung".
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