Join Date: Jan 2005
Location: Toronto, NYC, somewhere on planet Earth
Programs: UA 1K, AA ExPlat, Hyatt Diamond, SPG Plat, Marriott Gold
Posts: 8,291
mdriess, welcome to FT!
As Economy plus is considered a premium section on UA, they will mos likely not allow you to sit in the area if your BP is not assigned there.
They will likely make an on-board announcement to that effect.
You do have the option (if not filled) to purchase an upgrade to Econ+ seating. You may have seen it if you purchased your ticket on the United website and will also be offered it upon check-in (web or kiosk). You can also ask the check-in or gate agents about the buy-up price.