Some new grad students may be confused about ``commitment''.
If you decide to attend a research group, then this doesn't commit you to working for or doing a thesis with a faculty member in that group. Nor does it commit you to continue attending if your interests change.
If you decide to do an independent study with a faculty member, then that alone doesn't commit you to working for or doing a thesis with that faculty member.
If you are housed in a research lab when you first arrive, and you are not supported as an RA, then this doesn't commit you to working for or doing a thesis with the lab coordinator, or any other faculty member who is associated with that lab.
If you make a commitment to give a talk or do a task then you must do it.
If you are being supported as an RA then you have a commitment to work for the faculty member that has the grant.
If you are being supported as an RA then you do not necessarily have a commitment to work on your thesis with that faculty member -- even though this is normal.
If you are supported as an RA or a TA then you should know the commitments implied by the department's Guidelines for Teaching and Research Assistants.
If you are supported by a Fellowship then you need to find out what commitments are entailed.
If a faculty member asks you to work on their projects or in their research group, you can say no if you don't want to.
If you have asked a faculty member to be your thesis advisor then this is a step that implies a commitment. Changing to a different thesis advisor is possible, but it should be discussed with everyone involved.
If you said on your application to the department that you were interested in area X but discover that you prefer to do research in area Y, then you are free to switch, just as long as you haven't made additional commitments.
If you have applied for the Ph.D. program, and have been admitted, then this is seen by the department as a strong commitment by you to complete the doctoral program.
Try to give timely warnings if you expect your situation/interests to change in any way.
Overall, for your thesis you ought to find out what you want to do and with whom you want to work. And of course, honor your commitments, don't keep changing your mind, act reasonably, and talk things over with your academic advisor if you have doubts.
firstname.lastname@example.org / Tue Mar 7 18:45:26 EST 2000