Very good suggestions from Gilmeyer.
If you have a look at the Playbooks, "Chapter 3: Transition" will probably the most relevant one for you.
There isn't any estimation tool because each project is different and might not require much work.
For example I don't expect your plugins to be an issue at all because they run on the server-side, so independent from the client. As for the JavaScript, I recommend that you test them. Depending on how they were developed, they could keep working without any rework.
Henry
