If this is a situation where both switches control the same lights, you can’t replace just one of them. This is what is called a “three way” set up in the US.
There are at least 8 different ways to wire a nonnetworked three way setup, but they don’t all work with networked switches. Networked switches are not wired the same way as nonnetworked switches.
So the first general rule of thumb is if that more than one switch controls the same light, you have to replace all of them if you are going to networked (zwave) versions.