You probably won't be able to correct the inverse taper by scarring, unless you take a BIG bite out of the trunk with a wood gouge or concave cutter. However, you'd also have some risk of dieback at the pruning site, as amur tend to do that, sometimes dramatically, at large wound sites, only to regrow over them agressively.
A better way might be to inarch graft or thread graft a longer limb above into the "necked" section of the lower trunk.