YOU REKEY the cylinder. Modify it slightly in a manner that accomodates the modified key if that is what you have to work with. Having a similar blank would give you more options.
Like I took a bunch of new locksets to a locksmith. He took apart the keyways and re-pinned them (that's the word I was looking for) Now certain keys fit certain groups for certain buildings and a master key operates them all. All done with inexpensive off the shelf locks I bought at trade price.
Why not just put an altogether different key set on? Or as others have suggested and/or done, install a master cutoff which they would need to look for.