As far as I know you have to remove the lower inner skins and the belly pan from underneath. The plywood floor is bolted to the frame and contributes significantly to the structural integrity of the trailer. My trailer has 3/4" plywood. I plan on replacing with about the same some time...
I think I am going to try to replace the sheet in the front of the trailer this weekend and hope to post some photos as I figure it out. I plan on replacing the screws/bolts as they seem to be pretty rusted from years of moisture/rot.
I think you can put pretty much any type of flooring in that you like. Recommend something light to keep the whole trailer light.
Guys like Wasachris, Top and Terry have been helpful either through messages or simply following their posts.
Hope some of this helps.