You can just use exec to replace your current shell with a new shell:
exec
Switch to bash:
bash
exec bash
Switch to zsh:
zsh
exec zsh
This won't affect new terminal windows or anything, but it's convenient.
you can try chsh -s /bin/bash to set the bash as the default,or chsh -s /bin/zsh to set the zsh as the default.
chsh -s /bin/bash
chsh -s /bin/zsh
Terminal will need a restart to take effect.
I switch between zsh and bash somewhat frequently. For a while, I used to have to source my bash_profile every switch. Then I found out you can (typically) do
exec bash --login
or just
exec bash -l