A lot of changes are preventative, corrective, or some kind of defect repair action; all require change requests. Additionally, baseline changes and scope changes require change requests. The PM can do minor changes without formal requests, but none of them should really impact the job baselines e.g. crashing a schedule.
The process sometimes vary, but it usually comes in as a change request, gets documented, is evaluated, the impact documented, presented to the CCB, and then finally approved or denied.
The impact analysis should come from the entire team and stakeholders, not just the PM. This provides a lot of input.
The level of authority granted to the PM could also impact which changes the PM can do unilaterally as opposed to brining everything through the process. This should be described in the Charter early on as to what te PM can do and what changes need to be brought through the ICC process.