Afaik only vip and donator can change usernames you can maybe try to ask theymos but the chance that he will grant the request is very low.
Why you have so many gaps in your posts did you bought the account?
theymos...? Like the legendary theymos on reddit, the same person :-)?
Gap you mean between posts? Just wasnt paying a lot of attention during the "boring" period of BTC (2014-2016 :-) ).
Yes, theymos is the same person, administrator of bitcointalk and moderator of r/bitcoin.
Coming back to the question, If you want to change your username, then you need to get donator tag by paying 10 BTC = 27,820 USD as of now.
If you are that rich to change your username on a forum, go on !