Go for it! The worst that could happen is your T decides to keep the original evening appointment. I'm a graduate student and I work on campus and coach a high school sports team so my schedule changes often. I've asked my T a few times to change my appointment time. I think she always keeps my evening time in her calendar but if she has an opening at another time, she almost always accommodates me. I've tried asking her to change a session a couple of days in advance (usually happens when I'm really struggling) and she respectfully tells me "let's keep what we originally scheduled for this week". And that's the end of it. You've got nothing to lose and two weeks is enough time in advance!