I am removing the plastic rail on a 1/700 Fujimi Kittyhawk and replacing them with PE as well.
Adding gun tubs or removing things that are attached really is not that hard. Sanding off all of the deck detail, filling the cat tracks with putty and scribing the correct "Steam" cats at the proper angle in a nice, crisp, straight line is what I consider hard. It is hard to make that look right. It also takes some tools, ie.. a dremel tool and some good jewelry files and things like that.
I got most of my experience taking a 1/800 Midway and chopping it up into three pieces, stretching it, and filling in the gaps to make it into a 1/720-ish kit. I did Midway, Coral Sea, and F.D.R. F.D.R. was a big project in that I had to chop off a lot of the deck and rework it. I think that they came out pretty good so I use those as references when I am looking at getting the blade out on a kit.
When you get ready to go, we will all be here to lend our two cents worth. And sometimes, you even get more than two cents worth !!
Good Luck,
Mark