I am shooting from the hip here, but IIRC, if you choose Advantage, you can't go back later.
Unrelated to your question, I gather that expats can opt-out altogether if they can show that they are participating in another plan and do not live in the US. I gather that they can return later and join without penalty.
But then again, I believe in Truth, Justice and the American Way.
"I am not in a hurry.
I am retired.
And I don't care how big your truck is."