In general the way to connect to a 1771-SDN is to connect to the DeviceNet network.
The common methods, in historical order, are:
1770-KFD (DeviceNet/RS-232)
1784-PCD (DeviceNet/PCMCIA, obsolete and discontinued)
1756-DNB (ControlLogix backplane to DeviceNet. Get on backplane usually over Ethernet).
1769-SDN (CompactLogix 1769-L32/35 or later)
1784-U2DN (USB/DeviceNet)
The modern method that's best-supported is the USB/DeviceNet 1784-U2DN. They're pretty expensive if all you're trying to do is reconfigure one drive.
Another method that's very slow but may do what you want is to use the 1771-SDNPT Passthrough driver in RSLinx Classic. It will allow you to connect to the PLC-5 through one of its ports and then use a special feature of the 1771-SDN to access the network.
Why do you think you have to reconfigure the 1771-SDN in order to use a new-firmware drive? My guess is that it's because there's an electronic keying error (Error code 73).
If the problem is just an I/O size configuration error (Error code 77) then you should be able to resolve it through the PowerFlex drive's HIM module.
