So, the printers are receiving serial input form the current switch? If that is the case, odds are they are not using all 25 pins on those connectors. You need to check the serial port pinout diagrams for those printers to be sure which pins they are using. If they are only using the usual pins, you should be able to either buy or build your own 25 pin to 9 pin serial adapter to take care of that problem.

The ethernet to serial is still a question though. Specs on your current switch would help. I assume that the Alpha system is sending the print jobs via ethernet to the DELNI switch, which is then using that data to select which printer to send it on to?