1. Sun Java was never the default in Ubuntu, you had to install it by hand.
2. This is planned, OpenJDK was always supposed to replace Oracle Java.
3. If you had a version critical environment then likelyhood is that you already turned off automatic updates.
There are some things that this might break, there were a few tales in that slashdot thread of things not running in OpenJDK but nobody seems to be naming the sepcific calls being used that cause inoperability.