Wolves and dire wolves are separate creatures
Dire animals are born as such; you might be able to breed dire animals from their non-dire counterparts, carefully selecting the desired traits generation after generation, but no individual is ever going to make the transition, whether it is an animal companion or not.
Oddly enough, druids are pretty free to change animal companions; unlike familiars, there is no XP penalty for losing or dismissing an animal companion. So a druid could relatively easily “upgrade” by dismissing the wolf and finding a dire wolf to replace it with. But it is not the same individual animal, it does not happen automatically, and the druid would have to find the dire wolf in the first place.
Dire wolves and other “powerful” creatures adjust effective druid levels for animal companion bonuses
To get a dire wolf animal companion, a druid must have at least 7 class levels. If he selects one, the druid is treated as a 1st-level druid for the purposes of the improvements that the animal companion class feature makes to the dire wolf.
That means that, where a Druid 7 with wolf animal companion would give that wolf 4 bonus HD, +4 natural armor, +2 Strength and Dexterity, 3 bonus tricks, and the Link, Share Spells, Evasion, and Devotion features, a Druid 7 with a dire wolf animal companion would only give 1 bonus trick along with Link and Share Spells to the dire wolf.
As the druid with the dire wolf levels up, his “effective” level remains 6 less than his actual level for the purposes of the dire wolf. Thus, he would have to be a Druid 9 (9−6=3) to give the dire wolf 2 bonus HD, +1 Strength and Dexterity, another bonus trick, and Evasion.
Dire wolves do not get some +11 bonus to trips; their total trip modifier (that is, the thing they add to the d20 roll when making a trip) is +11. That comes from the dire wolf’s +4 size modifier for being Large and its +7 Strength bonus. As the dire wolf increases its hit dice (for instance, because it is an animal companion to a druid who has leveled up), it may improve its Strength (+1 to one ability every HD that’s a multiple of 4), increasing the trip check. (Animal Companions do not increase in size from bonus HD, so the +4 bonus is fixed barring magic to increase its size temporarily.)
Animal companions also gain feats (for every HD that is a multiple of 3, same as for any other character), so it could take, for instance, Improved Trip for another +4 (or, at least, could if it could find a way around the Int 13 requirement on Combat Expterise).