I second Russ' recommendation. Series Five components are only available through dealers in obsolete hardware. As time passes, they are going to get harder and harder to obtain.
If you have to convert in stages, I think this will work. Install a Genius Bus Controller in the Series Five and use it to communicate the I/O to a bus controller in the new PLC. With that architecture you should be able to convert from Series Five to 90-30 or Rx3i I/O modules on a planned schedule. The drawback is the need to buy a Series Five and a 90-30 Genius Bus Controller for no other reason than to keep you running with the old I/O modules.