Yes, that's what the G in TAG represents - Guarantee. Of course you will need to meet all of the contract requirements of the TAG up until your last day in CC (these are usually like GPA reqs and coursework completion, and you may not be guaranteed your selected major). Once you submit the TAG (you have to meet certain reqs just to even do that), then it's kind of a continuing process keeping the UC up to speed on your academic progress until the year is finished out. It helps the UC ensure you don't blow off any final obligations.
BE AWARE: TAGs, especially at UCSD, are becoming increasingly competitive still. Some requirements have recently been raised and may continue to be more stringent. For example, the GPA req for UCSD is 3.5.
IMO, if you got accepted to UCSD from HS, then as long as you really strive to perform the best in your studies, then you should have a shot at UCLA as a transfer. Of course there's no telling what the next 2 years (or less, or more) can hold in terms of the competition.
I think De Anza would be a great school to pursue a UC. I just took my placement tests there today
Try to join the honors or a sport to gain priority registration and save yourself some headaches.