Seems as if the answer lies in part to their strength, and hit points. Per
this archive provided by Stratics, the current range should be 721-760 STR, whereas prepatch exceeded that range. HP as well, but I don't know the exact range. According to
TamingArchive, there are a few other characteristics to check out:
On February 1, 2001, the "smooth skinned" version of the wyrm was discontinued. This wyrm was later reintroduced. White wyrms tamed after Publish 16 (July 2002) will also eat gold, ones tamed prior will not. Gold will restore a portion of the wyrms stamina depending on how much you feed it (Number of Gold Coins - 50 = Ammount restored).
There is also a page dedicated to
one specific pre-patch WW, which shows how many more HPs they had.
According to
this thread, pre-Publish 16 WWs also spawned without eval int, which I forgot about. I've played since '99 but I don't think I got into the major tames until after 2002, I'm not too sure. Took me a long time to raise my tamer, just wasn't into grinding it out, haha. I do have a prepatch frenzied ostard though.