Allright, so your siftmod doesn't have an uninstall option.......... So, extract the slayers package. You should now have an ISO file. Extract that one with craxtion or qwix. Now search for the retail C: files (I have no clue where they are located exactly, sorry. You gotta search for it). I you have those files, FTP to your xbox.
first delete your old MA savegame from your xbox (somewhere in E:\UDATA). Then simply FTP krayzie's savegame installer over to the udata folder (unzip the udata.zip and ftp over the numbered folder(s)). You can do this, because the old savegame is not needed to uninstall the softmod. When I told you to copy kryzies to the memorycard is was implying you would need your old savegame to uninstall the old softmod. But you don't, so you can actualy put krayzie's on BEFORE you uninstall the old one.
Now, the following part has to be done WITHOUT turning off your xbox inbetween:
1. Delete everything of the C: partition of your xbox.
2. FTP the retail setup from slayers over to your C: drive. Your C: drive should now look like this:

if not, DON'T turn your xbox off. If yes, you can turn off your xbox, turn it back on, and load MA.