You can fly the -400 with one winglet removed but for some strange reason, not with two!? The removal of a winglet is not a “dockyard” job , a few pannels , bonding leads and some BIG pins. But it’s a big lump and high up so you need a crane and high access equipment to get at it. I suspect that this is the reason that Boeing certified it to fly with one still fitted rather than the logical symetrical approach.

Rgds Cking