Recently, India tested Brahmos to 800-kilometer range and that may give us some clue on what is going on here. 400 kilometer range air-to-air missiles are nothing new such as Russian KS-172 which is in serial production and is called anti-AWACS missile for a reason. But since Indian SU-30s are designed specifically to carry Brahmos missiles, one can totally see a possibly 600-800 kilometer range M=3.5 missile to be used against AWACS when proper targeting is provided. Notable, on these distances AWACS wouldn't even know what his it. Moreover, a smaller version of such Brahmos doesn't need a rather massive 300 kilogram warhead of its anti-shipping variant which automatically avails an additional volume for fuel. Also, a complex electronics required for control of anti-shipping version of Brahmos, including its very complex flight profile, especially on terminal, should also reduce missile's weight in air-to-air version. Finally, the drag at the heights of few kilometers at which most AWACS operate is radically reduced when compared to a low (sea-skimming) profile for anti-ship versions.
The question, thus, is next--who, or, rather, what will provide such a targeting. Well, it is 2020, folks, not 1980 and things changed rather dramatically and a variety of means exists today to make such targeting possible especially since most air-to-air missiles are shoot-and-forget systems which need only initial position of target, plus different variants of CEC (Cooperative Engagement Capability) exist in advanced militaries. It is also obvious that such a Brahmos will have an active radar seeker with a good range which allows missile to conduct final (terminal) refined search and lock on a target. I'm telling ya, this whole thing is getting out of control, next thing you know--hypersonic missiles with the ranges in thousands kilometers... oh, wait! Wink, wink. It is jungle out there, folks, I'm telling you.