I got accepted to PhD program, but I am not sure with whom I want to work. Is it bad to change advisors? I got accepted to a top PhD program. I did not expect this to happen, but I am quite happy about this. A certain professor A voted for me (e.g I was accepted to a program because he liked me during the interview and voted for me). He expects me to work with him when I arrive. But, I am not sure if I want to work with him. I know nothing about the area he works in and most of his students did not join academia after graduation (which is bad for me). I would like trying to work with professor B, but I am not sure if this will work out either. Professor B might not want to work with me, or I might fail in his lab (the work that professor B does is harder and expectations are higher). So I afraid that if I try working with professor B - how will I explain this to professor A? What if I will not succeed with professor B, then most probably I cannot expect to work with professor A again, since nobody wants to feel like a back-up option. 

How does it usually work for students in PhD? Can students easily try working with different professors and then choose the one they like? Is switching advisors/ coming back to them easy? Or most often students stick to the professor who accepted them?

Country: US, field: CS
[Background: I'm a full professor in a top-5 American CS department.  My answer is skewed toward standard practice in theoretical computer science, but I regularly give the same advice to all incoming PhD students in my department.]

As others have said, practices vary in different departments, and in different sub-fields within each department, but as a general rule: It's your PhD.  Of course you can switch advisors!

Professor A voted for your admission, but he did not admit you.  The department admitted you.  Prof A's vote does not oblige you to work with him at all.  It's natural for Prof A to anticipate that you will work with him, but he has no right to demand that you work with him.  (These are all shadings of the word "expect"; I'm not sure which meaning you intended.)  If you prefer to work with Prof B, you do not need to justify your preference to Prof A.  (You should still communicate your preference to Prof A, of course, especially if he asks you about working with him.)

More generally, you should never feel constrained to only work with one professor, just because that professor happens to be your advisor or happens to be paying you.  Nothing prevents you from working with both Prof A and Prof B, provided you are meeting the obligations attached to your funding.  (Exclusivity is not a reasonable obligation for a research assistantship.  Progress, yes.  Good-faith effort, definitely.  Results, sure.  Exclusivity, no.)

In my department, faculty in my research area strongly encourage entering PhD students to work with multiple faculty — in the first year, to help everyone make more informed advising decisions, and in later years, so that students develop a more diverse research portfolio, and to provide a backup in case research with one faculty member doesn't pan out.  Yes, even the students I am supporting from my own grants.

All that said, it's best to approach the situation under the assumption of good faith.  Assume that Prof A and the other faculty in your new department have your best interests at heart, and approach them to discuss options under that assumption.  Be clear with Prof A that you are not sure you want to work with him and why, and ask for advice on how best to meet your professional goals, including approaching other potential advisors.  If Prof A objects, or tries to guilt-trip you into working with him, or tries to convince you not to even talk with other faculty, then congratulations, you've identified someone who does not have your best interests at heart.  Best to learn that early, before you waste your time working with them.