you could (i used Xcommander to do this)
1. backup the evoX dashboard
2. copy the XBMC folder over to the C:\ partition (makesure the default.xbe is in the root of C)
3. rename the default.xbe to whatever the evoX.xbe was ... i think its evoxdash.xbe, but dont quote me on that
this should work, but i wont recommend it , as if you screw up your xbox it will be a pain in the arse to recover it
maybe use norton ghost (or xboxHDM) to backup your HDD before you try it