You would qualify for the SDC (for free) if an S fare is available on the 777 flight beginning at 3:05 (the time of your original flight). Seatcounter has been under maintenance for some time and I do not have the KVS tool. You do have a reasonable shot I would think if you do get switched to the flight in the 3 o'clock hour since the 15 seats will probably be given away between T-24:05 and 23:57, or whenever the bulk EUAs run these days.

Also on the 777 there is a higher no show factor in F given the increased seat capacity. If an S fare is not available 24-25 hours out then you will probably be stuck waiting for the fare buckets to open up at 3 hours, which will be of little use for upgrading since these EWR-IAH flights usually have a lot of elites. Hope this helps and good luck!