When using the Blue-Green method, basically nothing gets changed. Instead everything gets replaced. We start by setting up a new environment – the green environment – and then cut over to it when we’re ready. Once we cut over to the new environment successfully, we’re free to remove the original blue environment. The technique is all about replacing components rather than altering components.
Check it out for a great explanation, not only of how true blue-green doesn’t jibe well with databases, but how to get it to work well enough.