Changing supervisors in graduate program Suppose that a student joins a graduate program with supervisor A. The supervisor has a bad reputation in the research group with a history of not caring for his students, e.g. professor goes to sabbatical on a regular basis and does not skype with his students, replies to emails from students in 5 minutes, does not help students with their coursework, expects students to comment on his research work, expects students to be independent, etc.

Incidentally, after a few months, he becomes more interested in the research interests of supervisor B in the same research group.

In this case, it is standard procedure for the student to be officially co-supervised by both supervisors A and B even though the student's projects are all supervised by supervisor B.

Now consider an alternative scenario. The student becomes more interested in the research interests of supervisors B and C.

In this case, is it standard procedure for the student to be officially co-supervised by supervisors B and C? In other words, does the student leave supervisor A completely?

How common is the second scenario in graduate programs?

Does the switch leave a bad impression of professors (in the research group) on the student?
The idea of a graduate (or PhD) program is to form the student into an independent researcher. This normally includes the liberty of the student to choose his own projects and research. On the other hand, the student also has to learn about funding, e.g. it might be that he only gets money if he works in As lab at least XX hours a week. This also counts as learning experience: If the student wants to work elsewhere, he might need to acquire new funds for it.

Regarding your question what of it is standard or not, that highly depends on field, country, university, even institution. Thus it would be best to ask your advisor, other students,... about how it is handled.