I understand the challenges of piecewise upgrades, but.. ugh. I won't make you listen to me tell stories.
What you've probably got is an SLC-5/04 controller with a slot-mounted 1747-SN Remote I/O (RIO) Scanner, connected to 1203-GD1 or -GK1 RIO Adapter modules to control the Bulletin 1305 VFDs over their SCANport connection.
This was a pretty smooth setup, once upon a time. Each drive would appear as 2 to 8 words of data in the I/O map of the SLC, depending on how you set up the "Data Links" via DIP switches and I/O size entries. You would command starts and stops in the Command word, and give a Reference between 0-32767 = Min Hz to Max Hz in the Reference word, and use the rest of the data table for your choice of feedback and control parameters, 2 at a time.
There were a handful of similar ways to do this: 1203-SM1 SCANPort modules, or 1747-SDN DeviceNet scanners with 1203-GD5/GK5 Adapters, or even ControlNet modules.
But with modern PowerFlex 520 series drives, the easiest interfaces are EtherNet/IP and Drive Serial Interface (DSI), since they are built into the drive.
Let's take a step back: what's your budget ? Are the drives all in the same cabinet ? Do you have access to the SLC-5/04 program to examine ahead of time ?
There might be a good argument for doing this with DSI (essentially Modbus RTU over RS485 wiring) and an intermediate DF1/Modbus serial device instead of trying to connect using EtherNet/IP, leaving that connection for Phase 2 when the SLC becomes a CompactLogix.