Metamorphosis is the way to go, or any other morph ability with the right adjustments. Be careful when using Meta as a morph spell as the Duration - Normal field actually determines how long it takes to transform (thank the
Weird Ability Guide for that one) and Duration - Hero determines how long you stay morphed.
I'm not sure if this is solved/answered yet, and I know it's a request, but there's not a lot in this thread to warrant much effort. Besides, messing around with the Object Editor will help you familiarise yourself with it; it'll be good experience.
And as a side note, upgrades
cannot be disabled via triggers.