While on the surface it doesn't appear that Tails has changed much other than gaining a little more confidence and aging a bit (he's currently 14). However, he has definitely changed in the (almost) three years he's been there. He's not as naive as he used to it and become almost hardened by the emotional turmoil of losing friends on the community and fighting Eggman, though he's still very willing to help others and make friends, even if it means they will leave him eventually. This creates a vicious cycle where he makes friends with someone, becomes dependent on them for emotional support, then lose it when they are let go. At some point this will likely break him, though that has yet to happen.

He still suffers severe guilt from the times he went into dark super forms and because of having had to face himself with his shadow, he's become afraid of himself. Of the uglier side of himself and what bad things he could be capable of doing. He even fears the possibility of having to face himself again than having his friends get hurt or killed (which as mentioned above, has seen quite a few people die on the comm and one person attempt to kill themselves), even though he has improved on some of his faults since then. You could say he's on the verge of a mental breakdown.

Basically he doesn't have the same happiness and innocence he once had, and is (almost) depressed.