If that is the case, the rest of the plane is likely to be equally packed and seat choice in Go may not be much better. That said, the extra leg-room in row 1 (even in the middle seat) makes it preferable to most other seats excl. emergency exit rows.
You cannot buy a Plus ticket and pre-assign in Go, as this is a separate cabin.
There are two options available for you: 1. ask at check-in/gate downgrade yourself or buy the Go ticket and pay for excess luggage.