I would drive the vehicle until it gets down to a quarter tank on the fuel gauge then have the remaining gas vented in the proper and safe manner. The tanks will then be safe to remove. The tanks are probably PST (Press Steel Tanks) and are heavy. The ones outside are the easiest to remove. The inside tanks are a bit more difficult. Check the file on this web site, there is a copy of the Crown Vic CNG publication that will give you an idea of how the thing is put together. It is best to have a professional do it.
Why not run the vehicle until about a month until the tanks expire? In the mean while look around for some newer tanks and have them installed.
It sounds like you got a nice car. Converting it to gasoline will cost some big bucks (gas tank, fuel lines, injectors, necessary changes to the wiring, replacement of the EEC V computer and probably re-programing). Then you may have to run premium fuel because of the 10:1 compression.
Hope this helps