You really don't want to get rid of CASH, you want XBMC as the dashboard right?
If so, that is a relatively easy thing to do.
FTP into the Xbox. You should see E:\Dashboard\default.xbe and Evox.ini if you have Evox as your dash.
Go to your PC folder that you have XBMC in and open the _tools folder. You will see two files needed. SHORTCUT by TEAM XBMC.xbe and SHORTCUT by TEAM XBMC.cfg.
Rename your E:\Dashboard\default.xbe to Evox.xbe
FTP to the E:\Dashboard\ folder SHORTCUT by TEAM XBMC.xbe to the Xbox renaming it default.xbe. FTP SHORTCUT by TEAM XBMC.cfg to the Xbox renaming it default.cfg.
So you should have in the E:\Dashboard folder:
Default.xbe
Default.cfg
Evox.xbe
Evox.ini
Default.cfg is written to look for XBMC in E:\Apps\XBMC. If your XBMC files are in a different place, you will need to put them there or edit the default.cfg file to point to where XBMC folder is.
Now you can restart the Xbox and XBMC will be your dash.