Torah Temimah cites Kerisos 27b.
One who designated his chatas may not bring it from one sin for another sin, as it is said “and he shall bring his offering for his sin” — that his offering be for the sake of that particular sin.
Diyuk of “al chatato” — the designation is locked to the specific sin; reassignment to a different sin is invalid.
A chatas animal that has been designated for one particular transgression cannot later be used to atone for a different transgression. The verse’s language “for his sin” requires that the offering remain tied to the original sin for which it was set apart. Attempting to reassign it voids the effectiveness of that animal for the new purpose.