I'm sorry, but you can't change your username without buying supporter tag for free first change. I can imagine the frustration of looking at old username, though.
One way to get around the limitation of using credit card is to have another person gift the supporter tag to you. That way you still be entitled with first free username change.
If you can't, there are some events and giveaways that rewards supporter tag. You should try participating these events and try your luck.
Alternatively, try contacting
accounts@ppy.sh for username change. This only applies if your username uses your legal name in portion.