Most small gun shops have a smith on site at least part time. An experienced gun smith can swap sights in less than an hour while maintaining a proper point of aim/impact. They may request that you leave the gun with them, but it depends on their work load. The last time I had a smith put sights on my gun I left it with him around noon and picked it up the same day before closing.
Sights are swapped out, so the old ones go and the new ones are installed in their place. They should give you your old sights back. They are, after all, yours.
My local shops charge about $30-45 to install night sights on a gun. I wouldn't be surprised to hear that shops give breaks on the cost of installation if the sights are bought from them, but I don't know about the shops in your area.
The industry standard seem to be Trijicon, but there are other quality makers including Meprolight and Heinie. I have Trjicon & Meprolight sights on a couple of my guns and have been equally satisfied with both. I imagine that one of the major manufacturers makes sights for your FNP.
I hope it helps.