You try a similar method to Curators ingenious way of extending the command sets. Command Sets are a one way deal the same as locomotors, so maybe his idea would work here.
He got the unit to spawn an invisble 'drone' the killed itself after some time. While it was alive, it gave the upgrade, then when it died it removed the upgrade.
If your first button created the drone, which in turn gave the locomotor upgrade, then your second button could trigger the death of the drone somehow, which coud remove the upgrade (is UpgradeToRemove = like the other drones use).
Hope you know what Im talking about!!
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