replacing decals

I want to replace the lettering on a SF loco with B&O decals. Should I just paint over the old lettering or remove the lettering first. What is the best way to remove the old decals? The loco is plastic not brass.
Fred

Fred;

Are you sure that they are decals? Most factory painted units are pad printed with paint. A white pencil eraser will remove most of the lettering without too much damage to the paint underneath.