Master key system

Depends on the type of lock.
If you are talking about all mortise type locksets then no problem, just pick a type of lock and keyway and then replace all the cylinders in the building to match.
Bored locksets vary, but often you can obtain replacement cylinders for these locks in other manufacteres keyways. Consult a local locksmith if you have these type of locks.
If you have more info available post it here and the honest hardworking types will offer some advice. Hope this helps.

if you don't know what a mortise cylinder is, look on the front of a glass/aluminum door on a store.. thats the mortise type, little over inch in diameter.
if these are ALL door knob types.. then IMO it TOTALLY depends on the AGE of the knobs..
50 year old Corbin, would be my base point, changing the other 2 to THAT, but, again, IMO, it depends ON the Sargent age and type of cylinder. If your Corbin is pretty new- then, I want cylinders in hand along with a good book.. SOME new ones can be all altered.. the Schlage IMO being the easiest to change. --Shiva--

If you want a solid answer, get a locksmith to survey the site and give you a detailed estimate. But the basic answer to your question is "You can probably just replace the cylinders on most of these locks."
Most reasonable-quality door locks (ie, better than Kwikset) can take replacement cylinders in a variety of keyways. In fact you can even get a few alternative keyways for Kwiksets, though gods know why you'd bother. A mortise or rim lock can be fitted for just about any keyway; key-in-knob and euro will be harder to match but in most cases you'll still have a wide range of choices -- up to and including key-controlled and high-security systems.
Some are more generic (and hence more flexible in this regard) than others. First step is to take a census of what you've got, then see what's available in replacement cylinders for them, then pick what you like from the intersection of those sets.
There may be situations where you have to -- or want to -- replace locks. But that depends on exactly what you've got, what it's mounted on, condition and so on. I've seen situations where better-than-average locks were mounted incompetently and were completely insecure as a result... and others where it was probably mounted properly but the door and frame had moved so much since then that the same kinds of problems arose.

Cyl's are replaceable for all of'em.
Newnsie

We don't really have enough information. Most of the locks with those brands have replacable cylinders. And some aftermarket brands of cylinders provide cylinders that may be able to get all the locks on a single keyway. Really have to be there to be sure.

Thanks to those who provided responses. I did ask a locksmith and he said the mortise locks would be reasonably simple but we have many varieties in-handle locks as well so getting all keyed the same without replacing more than just the cylinders is not possible. C George

Some key-in-handles take a fairly standardized cylinder and could be changed to a different keyway fairly easily. Others can't.
Of course it's still possible to do this selectively -- replace just cylinders where that's possible, and replace locks entirely where that can't be done -- so you're probably not looking at replacing every lock on the site. Again, a real quote requires a real site survey.