There are no way to configure CYPD3125 to CYPD3123 since the firmware of CYPD3123 using USB bootloadering, but CYPD3125 using I2C bootloadering. So that the firmware of CYPD3123 cannot move to CYPD3125 smoothly by only change the device part number in Creator. Meanwhile, CYPD3123 is DisplayPort sink role, CYPD3125 is DisplayPort Source role and CY4531 is hardware design as DisplayPort source role.
If you are only need to evaluate PR_SWAP, you do not need configure CYPD3125 to CYPD3123, you could test it on CYPD3125 directly with CYPD3125 notebook firmware. You could try below simple code to initialize PR_SWAP with CYPD3125/CYPD3123.
dpm_pd_command (port, DPM_CMD_SEND_PR_SWAP, NULL, NULL);