I have been having a similar discussion with one of my players who wanted to receive bonuses to his spell casting from freeing his 'dark side' that is normally repressed through the use of an amulet. Human Form didnt seem to apply since he still had his usual spell casting powers and he didnt want to purchase Demonic Co-Pilot since he was low on refresh. We could have used the Beast Change power but he already had his conviction and discipline at +5 which is the cap for the game we are running.
The solution we came to was that he take the amulet as an aspect, a focus item and when wanting to receive a bigger bonus 'use' it as a self maneuver placing a temporary aspect such as "In-tune with my Darkside' on himself with a free tag. Further bonuses available from tagging his trouble aspect and the aspect from his amulet. The theme is preserved with some narrative colour. He "takes off the amulet" or attunes himself with it as a free action when using it as a foci. He "mentally prepares himself" for giving into his darkside further when using the manuever (much in the same way as when Harry had his conversation with Lasciel)
Just thought I'd chime in with this in case it might help.