How to change init from sysv to systemd

[Bob Bernstein <poobah@ruptured-duck.com>]

I encountered an error during the install of a deb which 
led me to the discovery that my current init system is 
still sysvinit.

The error was:

/sbin/init: invalid option -- '-'
Usage: init {-e VAR[=VAL] | [-t SECONDS] {0|1|2|3|4|5|6|S|s|Q|q|A|a|B|b|C|c|U|u}}

Thanks to the wiki:

https://wiki.debian.org/Init

I determined that in all likelihood I am still using 
sysvinit, which circumstance was responsible for that 
error.

Problem: The wiki article suggests how to switch from 
systemd to sysvinit, but is not forthcoming as to the 
converse change, hence my Subject: line.

How should I approach this change with an eye to maximum 
safety?

Thank you


-- 
Man is essentially a dreamer, wakened sometimes for a 
moment by some peculiarly obtrusive element in the outer 
world, but lapsing again quickly into the happy somnolence 
of imagination.
                                    Russell