Can you change a brush to a different kind of brush? for example change a smudge brush and make it a

Select the brush tool first (or another tool such as the smudge tool if you prefer) then use Ctrl+Alt click to select the brush shape.

That works for occassional use of a new shape but if you want to use a shape regularly do the above steps then save it as a new brush preset. When you save it you choose whether to include the tool (brush, smudge etc) in the preset.

Dave