The worgen is not exactly a prime example to use, considering you still look like a furry anthro wolf 9/10 you're engaging with world content, and 10/10 times when you're engaged in combat. A "forsaken blood elf"-- and, just to be clear, I'm talking about the straight-back blood elf model, used for Dark Rangers and Sylvanas, here-- would not suddenly and immediately turn into a "forsaken human" the moment they get into combat. They'd remain as a blood elf the entire time.
Just to be clear, as far as my position goes:
• Want to make "forsaken blood elves" into an allied race? Fine by me.
• Want to make "forsaken blood elves" look like the Wretched? Fine by me.
• Want to make "forsaken humans" look like actual blood elves, only pale skin and red eyes? Then we have a problem.
Well, yes, you can. But here's the thing: your human character still looks like a human character, regardless if you look european, asian, or african. A forsaken character would no longer look like a forsaken anymore if they look like a blood elf.
You're making a false equivalence, here. The player character changing race when they switch factions is not something that exists within the lore. Thrall wouldn't suddenly become a human if he decided to defect to the Alliance. Hell,
Renzik "The Shiv" is an Alliance goblin and didn't turn into a worgen or gnome when he switched factions. That's a game mechanic.