The standard way to change your login shell is with "chsh". In theory you should be able to do:
$ chsh -s /usr/bin/zsh
and the next time you login, that should be your shell. "man chsh" for more details. I say "in theory" because I've never tried it on toolforge. I can't see any reason why it shouldn't work, but....
And also about the proper way to change the shell for tools accounts.