I just checked. In the latest patched version of vanilla civ4, warlords and BTS, the seal doesn't upgrade to a mechanized infantry. The patch can be acquired by the automatic update function inside the advanced menu of the game. If you got your game from STEAM instead of a shop, then you should also get the patch through STEAM.
I downloaded it...and I guess the fact that I ran Vista made it not work....so everything go bye bye....
Edit: So I reinstalled and went back to download your patch and an error comes up during extract.
Edit 2: So I fixed that, but now I want to do the auto update thing and I'm scared it will all screw up agian. Last time it said something was missing