Well, if you can't get something to work following the way you want it to work, try to think of other possiblities.
In this case, you should actually be able to switch locomotors upon switching of weapons (in your case deploying). Just make sure that the behavior is triggered by the weapon switch, but has a LocomotSetUpgrade (or whatever is used for it...can't do it by memory alone).
And if that still doesn't work, get creative: give the unit a behavior that creates an invisible object (OCL). And once the OCL is created, you can make that the trigger for the new locomotor on the unit that "deploys". You can add a behavior for a lifetime on the newly created invisible object to ensure its death basically immediaterly after it's ben created.
As such, the object won't show, can't be selected, won't trigger a unit lost (if it is of the KinOf = SYSTEM) and it dies instantly. It won't slow the game down at all (seeing as nothing is actually created, except for a new behavior that is triggered to switch locomotors). Could be an option....
