When I originally installed my v6.7 host, I recorded my 25-char key in my password vault. When I had to rebuild my instance, I used the same ISO boot disk to install 6.7 from scratch and I was able to apply the key from two years ago when I first got it.
Not sure if you have that old key, but for anyone else coming to this thread, it will work using the old key. I save everything all the time for situations like this.