I am a new person now. I am friendlier, more confident, charismatic, and a tad alpha and all my friends in university see me this way. However when somebody that I use to know comes along, I seem to slip into my old submissive, nice guy self whenever I am around them. I worked really hard at improving myself and these people just kill my state. Is there a reason for this and a way to fix it?

Start saying things like "I'm not like that anymore." Make it obvious that you are a changed person from who you once were. Talk about how "the old me" would've done such and such, but not the new me. Also, be prepared to punish your friends whenever they try to dog you out or pressure you into reverting back to the old you. Get pissed at them, become confrontational, and threaten them if they continue to say and do things you don't like. This will teach and train them to behave a certain way around you, and to embrace the person you are now.

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“Nothing is impossible, the word itself says 'I'm possible'!” ~Audrey Hepburn

Thanks for the advice, I guess I will have to do that. Most of the old people I meet are actually pretty quiet when I am around new people. The really influential ones are not really my friends, but guys I knew in highschool who always did not like me because I was in the smart crowd (ie. not popular). These guys are becoming friends with my new friends at university and really trying to mess up my social status. How do I deal with that? The friends I have now are standing up for me, but when I am meeting new people I fear this bad rep might make it more difficult. On another this is good news for me it shows I'v progressed so far that 've finally got what those guys had in high school. This shows that university is a whole different ball park.