I am unresolved on this Idea.
As for changes in general, I will have to side with the give classes flavor sooner argument.
Look at the new necromancer. Its a nice change once you get to the change.
But early necromancer levels are so repetitive.
c blin, c chi, c chi, c chi, c chi, (come on DIE!), c chi
Would have been nice to get a second low damage spell, like poison blast instead of poison. Poison damage with a chance to poison or something.
On Topic: I Like Variety. I like RP. I like change too, but charm person allows for RP that summoning some powerful beast would not allow. And what would your change mean for the Orb Cel? How would it work then? Would we the orb be gone? That's not exactly RP friendly. I am not attacking the Idea, as I never charm the right creatures anyway, and with your change I would be more likely to make a smart choice. I just wonder if there is not a compromise.
One of my favorite things about some classes is the paths. Why not path the dark knights, one path would be as it is now, one path would have other newer options. This allows adventurous players like Mya to find that epic new guardian, allows all those 'top in class' DK fans to continue their winning way, and also at the same time may inspire some other players to try a class they formerly struggled with because of a new set of skills/spells. Just the humble voice of reason.