I don't think they will change your username immediately I'm sure they are busy both Cyrus and theymos but there's other way to change your forum username I'm sure they are going to change it immediately however, you can't maybe able to afford it.
Read this below posted on unofficial forum rules
I don't know if 50BTC is the fixed amount to become a VIP member maybe try asking theymos about it.
Hahaha that’s too much money to spend for changing one’s username on a online forum. You quoted a post from 2014, the price of 50bitcoins then and now are not the same. OP wouldn’t even consider such an expensive option.
OP don’t expect your request to be accepted immediately, keep sending pms to Theymos occasionally and if you’re lucky he will accept. As far as I know, he has accepted quite a few of these requests in the last couple of years so keep trying.