1. removing old lettering; 2. re-decalling; 3. Applying a dull coat.
Removal is TRICKY - often silk-screened 'inks' are applied over paints, and re-painting + clear coat is needed prior to an all new decalling.
Decal manufacturing involves photo-reduction and ink colors and sizes may vary from original artwork.
Try or Walthers or Microscale's Solvaset first.