1) Create a custom ability, "Effect - Instant" and name it "Siege Mode". Check Tutorial Submission for one of my tutorials on how to create custom abilities and apply them to units.
2) Create two units, both called "Siege Tank"s. You can edit Prefix's and such so you don't get confused as to which ones are what.
For unit 1, use the mobility mode siege tank actor,
For unit 2, use the siege mode siege tank actor.
3) Create a trigger that goes off when "Siege Mode" is used. Actions would be as follows:
- Create 1 (Siege Tank (Siege Mode)) at (Position of (Triggering Unit)).
- Remove (Triggering Unit)
The good thing about using triggers in this case is it will go off immediately and it will not take time to use siege mode.
If you want a short pause, use the "Wait" function, place it in front of the first action in the trigger, then change the timer to your pleasing.