Forum:Megaroid City/ with Stealth Union?

Scenario: I control a Megaroid City in my Field Zone and a Super Vehicroid - Stealth Union. I attack with Stealth Union, and its ATK becomes 1800 per its effect. However, during damage caluculation, I send a Roid monster from my Deck to the Graveyard so as to activate Megaroid City's effect, then switching the original ATK and DEF of Stealth Union.

What happens to Stealth Union's ATK in this situation? Does it become 1500 or 3000?