If you have loads of old kill files, you may want to convert them into score files. If they are “regular”, you can use the gnus-kill-to-score.el package; if not, you’ll have to do it by hand.
The kill to score conversion package isn’t included in Emacs by default. You can fetch it from the contrib directory of the Gnus distribution or from http://heim.ifi.uio.no/~larsi/ding-various/gnus-kill-to-score.el.
If your old kill files are very complex—if they contain more
non-gnus-kill forms than not, you’ll have to convert them by
hand. Or just let them be as they are. Gnus will still use them as
before.