I can't stop you.

I have no idea what the changes would be. The eyes of the dark feature was the important feature from shadow magic. if that can be retained with a different subclass, that might be interesting.
Hey!
So looking at Escella, the key features I see are: attacks with a Blasting spell (currently Eldritch Blast) and rapier; Shadow theme/superdarkvision.
That can be done most easily as you have it, with the Xanathar’s subclass (Rogue-1/Sorc-6).
- Rapier attack can then do 1d8+4+1d6 radiant+1d6 sneak attack (with adv on subsequent attacks)
- Sorcerous Burst does 2d8, with a choice of damage types, but not the two beams from Eldritch Blast. Eldritch Blast is hard to pick up without 2 levels of Warlock.
- Sorc 6 now gets the Hound of Ill Omen, a shadowy wolf harassing a single target.
- If the flute is important, there’s a cool musician feat that can give Heroic Inspiration to some of the party members every rest.
She'd be stronger with Rapier, less effective with blasting, and now have the shadow-wolf.
If you wanted to do this PHB 2024-only, I think you'd be looking at Warlock 7 or Rogue 1/Warlock 6 (In another game I updated a Cleric and he became a Warlock too, to my surprise):
- you'd have Superdarkvision, and could become invisible in shadows at will
- Eldritch Blast: 1d10+4 (x 2) [as before]
- Rapier 1d8+4 (x 2) [+1d6 sneak attack 1/turn if Rogue-1/Warlock-6] [improved: two attacks, not one]
- Enchantment and Illusion spells could have "free" subtle spell.
So: I'd say there are three options for you:
a. stick with what you have, and update as 2014 character.
b. a soft-update, using 2024 chassis and the same subclass, and get the shadow wolf.
c. radical re-vision, replicating many of the the effects on a Warlock chassis.
you can do (a) yourself; I'm happy to help with (b) or (c). Let me know, and I hope this helps.