Machine a 81 / 9 degree chamfer in the 10mm flange.
If my tired physics/maths brain is correct with right angle triangles, then that will step it down from a 46mm on the front side to a 42mm on the back side of the plate.
Weld on 42mm tube and then chamfer the other 10mm flange the same 81 / 9 degrees from 42mm to 38mm.
This is assuming you haven't had the flangess made yet or bought the bends.
If you have had the flanges made then investigate inserts to achieve the same effect as what I suggest, plus you can have them machine a small flange on it to sleeve inside the bends.
If you have bought the bends then buy some new ones.
All impractical and spendy but the best way to achieve whay you want, all in my opinion of course.