game crashes when i upgrade back to its previous unit

[Gankme]

hey im trying to make some gates and i have them as two different buildings and they upgrade back and forth between open and closed
now it works just fine when the closed gate upgrades into the opened gate but when i click to upgrade it back to the original the game crashes saying there was an access violation in memory how can i fix this and over come this is there any key points i need to address when upgrading to buildings back and forth between eachother?

 

[SomeGuy333]

you should double check to make sure everything is set correctly.

 

[WastedSavior]

instead of using upgrades, use a replacing trigger with 2 dummy abilitys

Event
a unit begins casting an ability
condition
ability equal to (open gate)
action
replace triggering unit with opened gate, using old units relative life and mana

(something along these lines)

 

[Chocobo]

An upgraded unit causes a crash when it goes back its original state. (Example X upgraded to Y and upgraded to X, crash!)