That is perfectly fine as long as you let your opponent know and make absolutely sure they understand.
If you couldn't do it then it would be hard to theme an army.
They should still be WYSIWYG though, if it is supposed to have a staff and a sword then the model representing it should have a staff and a sword too.
Dark Angels 1750 win/loss Record
Win - 23 (Belial helped wreck Ryan's puny Eldar)
Loss - 13 (Brice's Necrons wiped me)
Draw - 2 (The 13th hold out in alpha take and hold)