So abilities in Warcraft 3 are hardcoded to do something specific. You can only change what the developers have made accessible about that ability. For the most part you are constrained to the abilities original design. If you want to go outside of that original design then you will almost definitely need to rely on triggers.
The Channel ability was made as a base for triggered spells, giving you access to some very useful options that almost no other abilities have:
Base Order Id: You've probably tried giving a unit two copies of the same ability before and ran into issues. The issue is that two abilities will conflict with one another if they share the same
Order Id. There's no proper solution to this problem, contrary to what the Object Editor says, you CANNOT change an abilities
Order Id. The exception to this rule is the Channel ability, which has this special
Base Order Id field that allows you to actually change the Order Id. But I think you're a little confused about this. This doesn't change the effects of Channel, Channel will never actually do anything outside of Casting/Art stuff - you're supposed to trigger the Missile/Damage/Buff effects yourself. All changing the Base Order Id does is allow you to get around the multiple copy issue.
Options: Allows you to modify certain behavior of the ability. You almost certainly want to check
Visible in here as otherwise you can't even cast the ability and there's better methods for hiding a button.
Targeting Image comes in handy if you want your ability to display an Area Of Effect ring when you're choosing where to cast it, similar to spells like Earthquake, Flame Strike, etc. My memory is foggy on the other options but they can come in handy as well and you should look into them if there's a certain behavior you're after.
Targeting Type: Allows you to modify the targeting mechanics of the ability (not to be confused with
Targets Allowed). This can be
Immediate (War Stomp),
Unit (Storm Bolt),
Point (Earthquake), or
Unit/Point (I think Shockwave can target both of these?).
Follow Through Time: This is how long the ability can be channeled for. If you aren't making a channeled ability then set this to 0.
Disable Other Abilities: I almost always set this to False as it Pauses the unit which often has unwanted side effects. But this could be useful for cinematics or unique mechanics in your map if used properly.
Those five fields are the main ones you need to focus on to get the most out of Channel.
It sounds like you simply want to copy and paste the Possession ability and modify this new copy to have alternate Art / Cast Range. Then give this ability to the AI's Banshee units. You could handle this Ability swapping through triggers fairly easily.
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Events
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Unit - A unit enters (Playable map area)
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Conditions
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(Unit-type of (Triggering unit)) Equal to Banshee
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((Owner of (Triggering unit)) is a Computer) Equal to True
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Actions
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Unit - Remove Possession (Normal) from (Triggering unit)
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Unit - Add Possession (AI) to (Triggering unit)
