3-way wiring - only replace 1 or both?

[JDRoberts]

This should help ( this is a clickable link)

FAQ: Zwave Switches 101 for Noobie (What's an Add-on Switch for and Do You Have to Replace All the Switches in a 3-Way?)

Brand new to Smartthings and trying to get some areas of the house “connected” ASAP. Most of the devices available are intuitive enough to figure out, but one area that seems to be a bit confusing is the on/off light/scene/etc. switchs that are available. I am mainly confused with the 3/4-way options, the add on switches and how these inter-operate with existing switches (if at all). My initial understanding is that I can replace a single pole switch with something like the GE Z-Wave Wireless Lighting Control On/Off Switch, if I want it to be able to dim the lights I need one that supporting dimming. If I have a 3/4 way lighting setup I need to get a zwave/wireless switch that is capable of supporting 3/4 way. However does this mean I can just replace one of the existing switches with…