Upon admission to the M.A. and Ph.D. in RSTC program, the director of graduate studies (DGS) and admission committee will assign you an advisor. The advisors whom you have been assigned upon first entering the program are just the DGS committee's best guess at a match between your interests and the faculty member's interests, expertise, and availability.
We want you to feel free to change advisors at any time and without any ill feeling. All you have to do is to get the approval of the new advisor, inform the old advisor, and let the DGS and program secretary know so that our records can be changed. You definitely should consider your choice of advisor carefully after you have passed the preliminary written and oral exams. At this time, you need to establish your dissertation committee and select an advisor who has the expertise and interest to see you through the dissertation stage in a timely manner and with a working style that matches your own.