Problem with Swaping Sovereign with a different ship

[Sonic14]

I was going to replace the Sovereign in-game with the Post Nemesis Sovereign. I followed the directions to the word, renamed the files, etc. But whenever i click on it, the game crashes, and I have to reinstall the game. Is there something that I'm forgeting to do to replace the sovereign?

[judgeking]

When I swapped the Sovereign for the Post Nemesis Sov, I got to play as the Post-Nemesis Sov.