I have a Benelli M1 Entry in transfer and has rifle sights. What is required to switch to the factory ghost rings? I understand that I can swap the barrel but how does the rear sight mount?

It is bolted on through threaded holes in the receiver, which you'd have to drill & tap. Benelli used a couple different rear sight assemblies. The earlier ones had a stamped sheet metal protective guard and were somewhat notorious for becoming loose. The later LPA rear sight has a machined base and is generally more solid. I think the receiver holes are different.

Do you have a source for a 14" barrel with the taller front sight for the ghost rear? They're not common.
I personally prefer the rifle sights. They are lower profile (which makes them less likely to snag on something) and harder to break.

That’s a good question. I’ve not looked for a ghost ring equipped barrel yet.