Yeah, the option is already available but buying a VIP membership is extremely expensive nowadays. The price for it is in fact 50 BTC not 20 BTC.
When the 50 BTC for VIP membership & 10 BTC for Donator status fees were created (on Aug 19th 2011), BTC was about $10. So VIP would cost you $500 and Donator $100.
Check for yourself here:
[...]
If you donate 50 BTC, you get VIP status, which allows you to set a custom title:
[...]
Donations page with more info: https://bitcointalk.org/donate.html
Going back to the original question, I'm thinking about how people could abuse this but since the profile is still tied to a user ID and you'll still see the trust rating for the user that just changed it's username, this won't affect anything.