I think describing it as switching from Oracle's Java to OpenJDK is rather inaccurate.
What they have done is switch the
implementation behind the Java Standard Edition API from Apache Harmony to OpenJDK's.
There is speculation that this was done either as a settlement or as a hedge in the ongoing Oracle vs Google case.
Also, while this switch can
potentially enable support for the newer Java 8 SE APIs like streams in future, for now it's still
Java 7 SE.
[just FYI: I edited the subject title to remove typos.]