The AR can't. ARs don't "call" anything. They raise the flag to recommend that the R call something.
Of course, we could build a system where the VAR makes a recommendation to the R to reverse the call, and the R would accept or reject that recommendation without an OFR--just the same as he accepts or rejects a flag. But there is something very different viscerally from accepting an AR flag on something the R didn't see, and accepting a VAR recommendation that the R was wrong. On the objective calls, that isn't a problem, but on the subjective calls, it would be a significant departure from the mindset the game has had for generations. Doesn't necessarily mean it's wrong or shouldn't happen, but its a much bigger step than VAR as it exists today. But it is also a bigger step on the slippery slope of opening up more to review with VR.
I still say throw the whole thing out--while it has certainly corrected somethings, I remain utterly unconvinced that the net effect has been to make the game better.