I just worked with Matrix Support on how to change the role of an existing Matrix user, and did take me some time to figure out that the "Select a User" drop down box, was the option that i needed to use.

This is not clearing laid out, especially since the existing Role is below this drop down box. It would make it much easier to understand/follow if this drop down box was to the right of the existing Role field, with the field label located above it.

Something to seriously consider.