In the vast majority of flights i've flown there has been no change of plan in the last couple of weeks. I'm not sure if IAH is particularly susceptible to last minute operational swaps. With the long haul aircraft i've observed they tend to know quite a bit in advance if they're going to be short on aircraft, so when a swap does happen it's usually been before check-in.

I know it doesn't help but swaps are possible at any time at any point prior to landing at your destination. You won't necessarily automatically be reseated together.