Commentary Par V 25-33

Given the special status of the freedom of our will and given the nature of a vow to God, made freely (no other kind is acceptable), the free will itself becomes part of what is pledged, i.e., one sacrifices the right to will any differently in the future without forfeiting the vow.  In a real sense it is a pledge to will no farther -- at least with respect to the matter of a particular vow.  The result is that one is not free to make substitution for what is originally promised, since that would be to replace the original sacrifice with something of less value, or simply to attempt to use again what had already been surrendered.

According to Bosco/Reggio (DDP Bosco.Par.V.26-27), Dante's rigorism with respect to the conditions for the substitution of that which was vowed is in polemic with the laxity in this respect of practitioners of canon law.  Aglianò ('voto,' ED.1976.5, p. 1152a) tentatively suggests the possible influence of the Spiritual Franciscans on this notably overstated position, far in excess, as Bosco/Reggio point out (DDP Bosco.Par.V.54), of what was allowed by Aquinas (ST II-II, q. 88, a. 10-12).