(Flash back to the old 4DC days)
While Air units can "land", and thus temporarily change layers, they cannot be permanently swapped. Thus locking them into a specific mode of travel. The Kykhu Oss, transformed via "replacement" method creating a new unit, passing the significant details, and then deleting the former. The biggest issue we encountered was saving / copying orders from one unit mode to the other. (It was possible, but only just) The AI especially had problems with this, as effectively the unit was deleted from play. With the newly spawned unit not able to be reassociated with the AI of the former. Optimus tried at length to find a work around without much success. Not that the AI was well understood at the time either, as many of the AI bells and whistles are internal to the game engine.
Note: The Cybran Naval walker got around this by having a workaround coded directly into the game engine. Thus enabling them to use the same unit for both Sea and Land layers. Far as I know of the unit just simply shifts from one animation set to another but otherwise its all seamlessly handled engine-side.
At one point we had a version of the Kykhu Oss that was comprised of two units. A visible and invisible unit that were both interlinked. We'd could easily control which unit was selected, visible and was subject to attacks. While this worked out great, it did have the drawback of using two unit counters. No matter what we tried, we were never able to spawn a dummy unit that didn't count against the players unit-counter.
In hindsight, I could of easily used projectiles, just having the "land" unit leap-frog from point to point. Effectively, we'd add a dummy weapon to the unit and have it target / fire on the location we wish to jump to. The projectile created would be invisible and would use a helper-bone to attach the firing unit to. On impact the unit would be removed from the projectile that's immediately deleted from play. Thing to watch out for here is that the units hitbox could impact the ground / water before it's helper-projectile, likely damaging or destroying the unit. This can be prevented by making the projectiles hit-sphere the diameter that's equal to the units largest XYZ dimension. Though also keep in mind that the games tick-rate is poo poo, thus what we perceive as an impact may or may not occur in the same game "tick".
@CDRMV : I'm betting that the JetPack mod will run into errors if the game is at or near its max unit count. Simply put, you'll be unable to spawn a helper unit into play if your unit counter is already maxed. That is unless the FAF folks corrected the unit count problems my 4DC team encountered.
You could try using the projectile methods as I've detailed above. Just use the units main bone as the weapon, while not requiring it to aim. Just add the requirement that the unit face towards the target before firing. The helper_projectile spawns in (fires), runs its Lua, to attach to parent unit. Once fired, you can remove the parent unit from the projectile as it's now inherited the projectiles velocity and direction. It should continue on to impact the target location, after the helper is removed. Keep in mind that many units don't survive being dropped from a transport, so it stands to reason that the JetPacks could lead to a units death from time to time. There is a Lua for this event as that you could override within your jetpack units Lua. The override would work only for that unit if done in this way, making it compatible with most everything else.
Edit: You may need to add a bone to your JetPack unit in Blender to get the projectile method to work. Reason this may be is that the "bone" must face in the direction you want the projectile to travel. Thus adding a "helper" bone to the units main bone via Blender should be pretty easy. Just have the units updated mesh in the /unit file with its bp. When you call upon the bone via the weapon blueprint and associated Lua, you'll need to reference the name you've chosen. Something Simple like "Helper" is usually good as this doesn't interfere with any of the other set bones. Otherwise you risk breaking the units animations. An alternate method is to create an "entity" that's attached to your unit. I think, this entity can in itself act as a weapon bone. Like i said, its been a long while. (10 years)
