I believe that a gateway will do the job. From what I understand, you need a gateway that is an Ethernet/IP scanner to Profibus slave. HMS has such a gateway, see:
There is no easy way to control Allen-Bradley Point I/O from a Siemens S7. (300, 400? In this case it doesn't matter, but it's always best to be as specific as possible.) The basic problem is that A-B and Siemens deliberately do not make their products interopable. Point I/O uses EtherNet/IP and Siemens will only talk ProfiNet.
There is at least one solution available; an AnyBus-X gateway can look like a ProfiNet I/O device to the S7 PLC and acts as a scanner to the Point I/O.
The easiest way would be over the profibus. AB is providing a point i/o profibus adapter. You can exchange the Ethernet module with the profibus one. That should work for sure and you can keep at least the I/O modules.
I dont know about the existing Ethernet wiring. How difficult it is to exchange it with profibus.
Its just an option you can think on. I dont have any better idea right now.