To change a ship texture (not just glowy parts) you need to change the base texture file(s), not just the ones with the -glow extension.)
Unless you edit a copy of the pof and make a new entry for that specific ship, ALL Fenris's will then look like that! (unless you're ok with that and don't care to use the regular ones at all no one will notice that.)
Simply open FRED tot eh mission you have your Fenris in and change teh name of the ship. Bingo you are done since you already altered the existing files... OR you copied the textures, changed the name by a letter and made a new ship entry and edited the pof to goto the altered textures so you can have BOTH old and new Fenris types in that mission...
Hope that helps you a little!