DASH

Here is what Wikipedia says:

Debian Almquist shell (dash) is a POSIX-compliant Unix shell, much smaller than bash. It requires less disk space but is also less feature rich. dash is a direct descendant of the NetBSD version of the Almquist Shell (ash)

From BASH to DASH:
sudo dpkg-reconfigure dash
and say 'yes' to switch /bin/sh link to point to DASH.

From DASH to BASH you can just run the following command:
sudo dpkg-reconfigure dash
Choose 'no' when it asks if you want to make sh symbolically link to dash. The same command can be used if you ever want to switch back to dash.

Well, Ubuntu switched to it for a default shell:
https://wiki.ubuntu.com/DashAsBinSh

The major reason to switch the default shell was efficiency. bash is an excellent full-featured shell appropriate for interactive use; indeed, it is still the default login shell. However, it is rather large and slow to start up and operate by comparison with dash.