The only such transition I was involved in was switching from TFS to Git, and there was no discussion. It was the obvious thing to do and for a while we needed to support some developers who are new to Git.

So, it all depends on the type of change you want to implement. Most people don't think much about a piece of software being open is significant. That's why the main selling point should be the product itself. Especially in organizations openness alone is not a strong enough argument.

But with individuals, it may help to inform people about FOSS instead of just suggesting alternatives ("Do you have a moment to talk about our lord and savior Stallman/Torvalds?"). If the individual doesn't understand or subscribe to the values, the switch may be temporary. My 2 cents. Hopefully others will come up with better tactics.