What is involved to replace the button with a lever type floorplate release on a FN? Brownells and others sell the lever but I'm curious what modifications need to be made to install it? Major machining or is it just fitting?

Hmmm might be a dead issue. Brownells has dropped them. Midway says no back order.

I have a couple if you need one, the only way I've ever installed them is when converting to a hinged floorplate. I prefer to make my own over the meier one's though.
Don

I've done one of the Meier's but I'll make my own next time. The Meier wasn't very precisely made and required so much work to make right I'm sure I'm better off making my own.

Now, to answer your question I need to know more about what you have. By "button" do you mean a non-hinged floorplate with extended release? You can do a lever without hinging the floorplate, and I've seen that on some pre-WWII German sporters, but it's just as much if not more work than hinging it.