This is easily done with the supplied conversion scripts.
We STRONGLY recommend you take the offered option of a backup first !
The process for conversion is :
- /scripts/convert2maildir 
take option 1 to backup mail accounts - /scripts/courierup –force
 - /scripts/eximup –force
 - /scripts/upcp –force
 - /scripts/convert2maildir 
take option 3 to convert mail accounts 
