Harpyon Urge

The chosen unit takes flying steps as if it had the airborne ability. If the unit cannot take steps (due to being immobile, for instance), it cannot move to the targeted minion.

See Moving to a Specific Location Via a Special Ability or Magic in the Codex.

Only check that the target is weaker when it is initially targeted, and when the effect first starts to resolve from the storyline. Once the effect is resolving and minions are moving, it does not matter if the relative powers change after that point.