E+ strategy for US *G flyer on likely overbooked E- flight

I'm confirmed on UA906, FRA-ORD, on July 30. As the seating chart below seems to show, almost the entire E- section for my flight is booked, while there is a ton of E+ remaining.



Since I'm *G, would it be a wise strategy if I:
- assume that UA will oversell E- on this flight
- not purchase an E+ seat at this time
- anticipate a free upgrade to E+ at the airport?

I have no experience with international UA E+, so any guidance will be appreciated.

Thanks!