Switching between different graduate departments within the same graduate school? How difficult it is to switch between different graduate departments within the same graduate school? And how can a student initiate the process?

Some have told me that it's easier than getting into a graduate department via PhD admissions. But I do wonder - does it depend on what school it is? (I'm at Brown). What other factors determine how easy it is to get into another graduate department? Would they look at your transcript/GRE scores/etc? I have GRE scores of 96th percentile Verbal/96th percentile Math/99th percentile Analytical/89th percentile Biology. My grades are weaker though. I'm currently a student in a Geoscience department and am considering switching to Physics or a Biology-related field.

Of course, one has to convince someone else to be able to take them on. Most of my experience is with Physics and Scientific Computing, however, and a lot of fields really value the skills that people in Physics/Scientific Computing have.
Maybe this differs between departments or schools, but in my experience transferring between departments is exactly the same process as applying from the outside.  If faculty in the new department know you and are familiar with your work, then you may have an advantage (or disadvantage), but there is no advantage from being already enrolled in another department.

If you want to do something interdisciplinary that could fit in either department, it's usually easiest not to switch (so you don't have to deal with applying again, fulfilling different class and exam requirements, etc.).  Instead, you can try to arrange a thesis committee with representatives from both departments.

If you want to start over in a new field, then the only option is to apply to the new department.  You'll need to convince them of two things: that you are changing departments because you now understand better what you would like to do (rather than because you are doing poorly in grad school), but that you will complete the new program rather than changing interests again in a few years.  This means you need a compelling story to explain the shift.