I just (re)installed IP the other day after playing with just the base game on my new computer. Of course IP is buggy and I knew it wouldn't be perfect. But I'm having issues with mermaids. They aren'...
Your current games won't use the new version of the world. You'll need to start a new game to test it. But you can still continue with the existing games it's just that nothing will have changed in them. The new world won't fix the mermaid problem as I have the problem and I'm using the new world.