I don't have a tutorial nor have I attempted this particular conversion but I suspect it may be more trouble than its worth both in effort and money.
Basically, the legs aren't too difficult and you'd just have to get rid of the extra folds and cargo pants pockets on cadian legs since mordian ones don't have pockets. The arms also don't look too bad but you'd have to file down the should pad armor plate to convert it to an epaulette. The chest is where you'd run into the biggest problem as the current mordian line from 2nd edition has multiple braids and awards but not visible flak armor on the torso. The cadian command box gives you two possible chests you could use (7 as is and 9 with some added decoration) as well as a single head with a plastic peaked hat (33) since mordians don't wear helmets either. You'd either have to sculpt something similar on your own or buy up alot from various online bits stores to get a proper sized force. Creed actually makes for a nice mordian commander since he doesn't have the typical cadian armor; if you do end up using the officer bits listed above for your rank and file, he may be a good choice for the army commander since he's a different pose/model. Hope that helps!
Automatically Appended Next Post: This blogger apparently went through trouble of turning cadians to praetorians (which are effectively mordians with a head swap)... that's alot of greenstuffing!
http://s4.invisionfree.com/Boot_Camp/ar/t9972.htm