Yes, when you faction change your mounts all switch over to the appropriate mounts that are equivalent to the other factions. For example with racial mounts:
Darnassus Sabres - Forsaken Warhorses
Exodar Elekks - Silvermoon Hawkstriders
Ironforge Rams - Darkspear Raptors
Gnommeregen Mechanostriders - Thunder Bluff Kodos
Stormwind Horses - Orgrimmar Wolves
Everything else switches over to the opposite factions equivalent (such as Frostwolf Howler switching with the Stormpike Ram).
There is a (kinda) full list here:
http://www.worldofwarcraft.com/info/fac ... /index.xml
I definitely recomend if you are going from Horde to Alliance you spend the 20 days (5 minutes a day so it isn't much) to get your Venomhide so you can own that gorgeous tiger!
Another neat tidbit is that the Woolly Mammoth, like other mammoths, has two different versions: one Horde, once Alliance. When you faction change, your Woolly Mammoth stays the same, AND you can buy another one to match your current faction! This bug has been in since they added faction changing, and it doesn't look like there is any move to fix it. So I'd also suggest getting both of these to up your collection.
