My first thoughts were there must be many such converters out there already, so I just cobbled this search string and there are many hits.

Convert 1080p video to 720p video

So here is an ironic youtube video of how to do it, http://www.youtube.com/watch?v=6Yde2vxVLA8, but there are many other hits on this search string. That one is meant to be a free web converter, so your start point 90GB may be a killer, you would have to have an off-line converter.