Honestly I don't know how that works. My understanding is that you cannot move existing ASL code under a different terms other than ASL unless you are the copyright holder. All modifications you done can be subject to whatever terms you liked. This seems to match what they said http://www.apache.org/foundation/license-faq.html#Distribute...

I think this important bit is sublicense v.s. relicense, where in sublicense case, you have to retain the original terms carried for the original code.

Now, Elastic relicensed the whole software stack under SSDL. So to integrate any parts AWS done, they need to license Elasticsearch back to some mixture of SSDL and ASL, which would be weird.