No, there isn't such option and it has already been asked
recently. Normally if you changed your username on the site,
all your galleries names should have changed as well...I
tried it and it worked. Did you see a confirmation message
after you changed your username? Did you do it in your site
profile? (not on the forums)

For now, I have changed it manually.

that's becuase you have to go into your fanfic profile
thingy and it should say "Change Fanfic Name" or something
like that. Just change it to whatever you want. same with
your fanart gallery. *if you have one