Aren't 12 year old daughters fun?
The trouble with children at this age is that they can be very short sighted, and not understand the kind of opportunities she would be missing if she passed on enrolling in this program. At this point, she's probably putting a lot of interest into her friends, and listening to music, experimenting with make-up and fashion, and perhaps she's worried that she will not be able to continue to pursue the things she's enjoying now if she takes advantage of this door that has opened itself to her.
She's already been identified as gifted and talented and no doubt has the academic scores to go with it, so if it hasn't been an obstacle to her making and keeping friends up until now, she probably won't have trouble with her social skills if she takes the opportunity.
Could you make an appointment with a guidance counsellor at the school to discuss with her the pros and cons of both staying in the program that she is in and joining the program that is being offered? Is this a one-time offer that may never come along again? If she tries it for a period of time that she agrees to (one or two terms/semesters) and absolutely hates it, can she re-join the group that she is in? Will she still be able to play the same sports and other activities that she is involved with now? What kind of homework load should she expect compared to what she is doing now? Is a move to another location involved so that she would not be able to spend time with the friends she has? Maybe once she knows a little more about what to expect, she won't be as anxious about the transition. The other thing you talk with her about is what her plans are for her future: She simply may not realize that this could be a stepping stone to other opportunities in the future... all she's thinking about is that she's 12, and it seems to her that her future is such a long way off that she needn't think about it right now.
Change is a very scary thing when you're 12. I remember not wanting to start high school, and being so anxious over it that I'd make myself sick with worry. Mind you, the WTS did a fine job of making me believe that I was going to be mobbed by drug addicts and boys with only "one thing" on their minds. Once I realized how tame it was in reality, it wasn't so bad after all.
Would your daughter like to penpal with another 12 year old? Mine is available.