I have about 30k skywards miles that are going to expire and I won’t have an opportunity to spend them.

I know the value isn’t great, but I’m inclined to transfer them to Marriott to at least get something.

My hesitation is how will I know “which” miles will transfer? Will my oldest miles that are due to expire go first? Or will they take me recent ones?

I don’t want to convert and still have miles expire - trying to avoid that.