Yes, the external 24 MHz clock can be connected directly to XTALIN pin with XTALOUT unconnected and firmware change is not required.
Please refer to the App Note for the external clock requirements.
Thank you for your helpful reply and link to the app note. I have a logic analyser based on the above chip with an Altera Cyclone IV FPGA. I'm pretty sure the FPGA is clocked from the Cypress chip/xtal so I'd like to change the xtal to a 3.3v txco for better accuarcy in the logic anlyser measurements.
Have a good day......