BT Pes 72b(26) :טריד אע׳ג דלא מיחלף מחלף אע׳ג דלא טריד‏ : (if ) he is pre-occupied even though (the sacrificial animal) cannot be interchanged, (or if ) it can be interchanged even though he is not pre-occupied
See the entire entry for ḥlp vb. to pass by, pass away; D to change s.t.