I was able to rebook GRU-EWR-MCO to GRU-IAH-MCO and kept front cabin seats for tommorrow night with no problems at all, after the EWR-MCO segment was CXed and pushed back to Friday. This is actually a major improvement over the original itinerary which had a 6+ hour layover and now has a 2 hr 20 min layover.

Many thanks CO!

Following previous poster's advice, I knew exactly which flights had seats up front and asked for them.