Cynthia, before thinking about centralizing or replacing where and what I repeat that according to me your first task should be to evaluate carefully camera by camera if what you have now is working or not accordingly to your needs and expectations. You know, us Italians are fond of antiquities and we always think more than twice before throwing off something..;))
Jokes asides, "needs and expectations" is definitely a relative point and it comes out from the answer to the question "what do I ask/need from my video surveillance system today?".
Try for example to get information internally about how many times it happened a critical event (and what is for the College "critical event" or "event-of-interest" it's definitely another focus point to recall and fix) and the video surveillance helped to solve/limitate/manage it? Or on the contrary, how many times "you" happened to say "Ah, if we had had more resolution that face/plate/detail would have been more recognizable..." or "Ah, if it had been more light...." or "Ah that camera was too old and flickering, if it had been better...." or whatever recalling to you if the system was effecitve or not in the past? And make camera by camera a check if it able to give enough quality of image (resolution, contrast, lighting, dynamics, ..), according to the specific environment it is put and the specific purpouse it should have.
After that you will have a report where you will have 500 results yes/no.. It may even happen that surprisingly you will read 50 "no" ans 450 "yes". In this case you may decide to think about centralizing all, by acquiring the analogue in a way or in another. But if instead, for example, you will have 400 "no" and 100 "yes" (or "yes, more or less.."..), so the solution is maybe actually to replace them almost all; but this, as written before, means first of all to make an accurate network infrastructure project.
Consider that anyway to make a transition analogue/IP has a cost. If you decided that anyway you will go to IP sooner or later, think well about maybe finding the budget to do it all now; because anyway if you buy now encoders or whatever to keep the analogue today, when tomorrow you will replace anyway all to go to IP these encoders will be useless then. So, it's like when you do a financing in a bank: you pay after, but more. If "tomorrow" (because of the report before) means 4-5 years, it has surely more than a sense; if "tomorrow" means probably next year, I think not and maybe the best solution would be to clench your teeth using this one that you have until you don't have the budget to do it once and all. IMHO..;)
Cheers,
Simone