Once you get aspect mastery which is a level 72 trait, you don't have to use transpose in your rotation unless you are doing level synched content below 72. You may use it after you kill a pack of mobs to then be able to use umbral soul if you were in umbral fire, but not during battle. You can cast the opposite element with 0 mp. So you can adjust your rotation around this. After your last despair/flare, you can go right into casting blizz 3/freeze.