Full convex BK2????

[Xr6x]

Seriously, what’s the best way to give a sharpened knife a full convex edge? Would it be to sand the current edge flat and start all over?

No, you don't need to sand the edge flat first. It's a tough one to explain see if you can find Jerry Fisk on you tube. It's best to see someone do it. He shows how on a vertical belt sander with a slack belt . Or it can be done with days of elbow grease with a mouse pad and lots of sandpaper look at Bladites post #15 a nice example of what it should look like when done . Using a ceramic rod will eventually get you convex edge .
Ceramic rod edge below . I didn't keep it going and changed it up with a DMT wet stone .

Dmt edge below

Another shoutout to 91 for sharing his recommended sharpening stones