You need to make sure EY operates a presence at the transfer desk, which they should as a major airline and many other airlines (like Asiana and Korean) codeshare with them.

You can transfer between terminals - just show your itinerary and you'll do a simple security check. HOWEVER, you cannot go back to T2. T1, Concourse, and T2 are connected by a train but they're very much only for passengers that are supposed to be there and they do check your itinerary/boarding pass. They do not like it when you go back and forth and always call some escort to guide you there blah blah.