Well I finally got onto these today. It was a bit of a pain it the backside but once I got the knack I finished them off fairly quickly. You can get really good accuracy just using the tip of the scalpel working from the top of the playfield.
It also ended up being a really good opportunity to adjust all of the switches beneath them as I went. I've found that its one thing to just swap everything from one playfield to another but ensuring everything works as it should when you do is what takes the time. Very satisfying all the same when you know you have it right. Many of these rollovers wouldn't have registered in years I'm sure.