You cannot do this
The Mystic Arcanum feature states:
Choose one Xth-level spell from the warlock spell list as this arcanum. You can cast your arcanum spell once without expending a spell slot. You must finish a long rest before you can do so again.
This requires to actually choose a spell of a certain level from your list, unfortunately the upcast version of a spell is not on your spell list and so cannot be chosen with this feature.
That said, houseruling this would likely not be game-breaking as most spells upcast to a given level are weaker than other spells of the same level.