Can my locks be re-keyed?

[Number21]

So, my '73 didn't come with keys for the doors. It did come with two seperate gas keys, an ignition key, a canopy key, and a lug key. My other '79 has door and ignition keys, as well as 2 more gas keys. And I have a Jeep that uses GM keys. In other words I have too many damn keys that all look the same!

So I was thinking...would it be possible for a locksmith to modify the lock cylinders in my '73 so that the keys to my '79 would unlock both? Or would I have to buy a new matching set of four cylinders and change 'em out?