Can a soldier lose a pay grade when going from reserves to active duty?

Yes they can, but if you do everything that is expected of you, you can get it back in no time.

Can they? Yes, they certainly can. Almost happened to me....twice.

It’s called a grade determination. It’s not an issue as an E4 but anything higher and you could have to give up a stripe or two. I saw a Reservist SFC come active as a SGT one time. Most people simply aren’t willing to give up that much rank to go active