It kind of depends on how well the old bed liner was prepped when it was installed. If it's easy to remove I would remove all of it before applying the new stuff. I did the roll on (Herculiner) on mine long ago then decided to remove it when I painted it a couple of years ago. I prepped the tub well before application, then cussed myself the whole time I was removing it. I had to use a couple of sharp wood chisels and work at it for days just to get the footwells/firewall/kick panels cleaned off. The bed floor I left on and just painted over the liner. If it wasn't prepped well it should pretty much peel off with a little work. Roughened bare metal or roughened paint for the new liner, if the paint is well bonded to the floor pan. Just like paint/primer the liner needs a good substrate to properly bond and last.
Mark.
