If you get one of the current mounts at the current price, it will switch to the new mount (same price) when the patch hits. The current mounts will be available for 5 [Champion Seal]s and a fistful of gold (400g at exalted, non-faction must be exalted to purchase the old ones).
The patch itself will be a minor patch before 3.2. No news on what they look like except they'll represent the faction's colors more accurately:
. . . faction mounts offered by vendors on the Argent Tournament grounds will have their visual appearances improved. You will find that the mounts being offered will better match the colors of the faction you chose to represent. (Nethaera)They call this an improvement, so I'm hoping it'll be wicked-spiffy and not just more basic recolors (though the white skeletal warhorse is A+ material, and you know how I feel about the raptor). But the rest look just like the normal mounts you get from city vendors and have no special feel to them.
So I think the changes will be a good thing, especially for the alliance (since theirs were the least impressive).