@P4RT1CLEM4N 

This is acutally quite easy.  Log into the Deco App and click on the add button.  Follow the procedure for adding a new Deco Unit to the account.  Once you have completed this will all of the new Decos, swap them out with the old ones, and that's it.  The New decos will be configured with the existing network settings and your devices will all reconnect to the new Decos with no futher actions needed.

@Carl Thanks for the reply and suggestion!  This sounds easy to add mesh nodes to my existing network. 

Quick question: I am using one of the Deco M5 "pucks" as my base router-- How do I swap out or replace the actual Deco M5 router unit with the new unit without disturbing my existing SSID/Password?

Update: I'm probably being dense here and I apologize if so :)

Bump.  Please respond.

@P4RT1CLEM4N 

Sorry for the delay I just got back into work for the day.  When you 'Add' the new Decos you are sending the entire configuration to each unit.  What this means is any Deco can be the base.  The only interruption comes from the boot up of the new base unit.  SSID/Password connections will all remain. 

@Carl Awesome and thanks a ton for the time and explanations!  Going to give it a shot this evening.

@P4RT1CLEM4N 

My pleasure.  Let us know if you need anything else. 

To provide a quick update: all went well when I added several new nodes to my existing network; as expected, the new nodes inherited my existing network information/settings.  Only minor hiccup was in changing my old unit designated at "Main" to one of my new units.  I found this thread that helped me navigate that process: https://community.tp-link.com/us/home/kb/detail/281 .  Once I changed the designated "Main" unit, I was able to remove all the old units from my system.  I am now using entirely new Deco units in my mesh network without having to change WiFi SSID or passwords.. 

Good luck and thanks!