What before-the-ending changes are there in the Extended Cut DLC?

Short answer - I don't think there are any changes before the final rush towards the beam.

Long answer - This page on the Mass Effect 3 wikia site details a lot of changes after the approach to the beam scene that constitutes the start of the outro sequence, but doesn't contain any information about changes before these scenes.

I haven't played through Mass Effect 3 from the start to confirm, however when I replayed the last missions after the Extended Cut was released I started from the beginning of the mission at the Cerberus base as suggested by Bioware themselves on their website but didn't notice any changes in the story up until the beam approach scenes, where rather than controlling the approach yourself the game took control to allow the changes in the Extended Cut DLC to be presented.

This page further details the additions to the epilogue cutscenes based upon your EMS rating at the time of completion.