I don't want to sound like a dick, but I can't agree with either of you. Your GF seems like she's too hung up on you changing any part that really isn't you, and you decided you need to change your name because that's what you're supposed to do, and being told by someone else.
I've been in your GF's shoes (in a minor way) when friends of mine changed their online nicknames and I too way too much exception to that. In retrospect very silly and just me feeling excluded and/or not liking change. So I feel justified in saying that it's not a very healthy standpoint.

You don't mention your own reason for changing your name. If it's down to casteism, I can respect that, but if it's something more akin to appearing cooler, your GF might feel YOU are being pretentious, rather than just disliking the name you happened to have chosen.

I wonder if it's likely about more than just the name, but what it represents. Maybe's she not so cool with you taking a serious step towards an acting career. Or at least showing you're willing to make major changes to accomodate that, and is worried how far that might go.