The Mobi2Mobi GUI can be used to change 501 to EBOK, but under Windows 7 it has a bug that truncates the printout from mobi2mobi - which often means that you can't see the current value of cdetype (exth 501). So the edit works, but I had to check it using the command line mobi2mobi (and the command line can also edit the exth value directly).
I just tried the Java Mobi Metadata Editor and it is very basic. The output filename is fixed, and to switch from PDOC to EBOK you have to delete the existing 501 and add a new one. It is very simple to use though, and it can also be used to check what the current value of exth 501 is.