Oh for heaven's sake...I guess some folks either don't think you can buy another layer of foam for putting other rifles in the case or think that the foam is expensive.
Yes, you can do the job with an electric knife.
But wanting to switch guns to be carried in the very same case is not an objection, to my mind. Just buy the top layer of foam...from Pelican for more $$ or just about anyone else (lots of sources) for less. Have them cut for each rifle you plan to put in the case, and Robert's your Mother's uncle.