You can just change out that one lifter if it is bad. Flat tappet would need to be broken in, roller just drop it in and go. Unless of course the flat tappet is munching into the cam but I really doubt that. Check the preload first since it could just be the adjuster backed off or was set too loose.
Simple check just grab a good size screwdriver and listen along the valve cover. It should show up pretty loud and clear if it's the valvetrain.