When can I sub out EU for NA???

You only have a single team comprised of both NA/EU players. You start them both at the same time; you just lock them in on different days. Once Eu is locked, those positions can't be changed until the following week. You can only start NA players in whatever role you didn't start the EU players in, until the next week.

Last time they locked in on the same days, so I assume after NA players lock in, you can fully edit your week 2 roster.