Hiya,
Just to check - you have got the correct firmware for the FXT009?
As you noted, I would also stop any running OpenAT application (AT+WOPEN=0) before starting the firmware update. I would also erase the application from the modem (use AT+WOPEN) to ensure that there is enough space in the modem for the download. It might also be appropriate to resize the A&D data area (again, use AT+WOPEN - check teh AT Command guide for exact usage) before doing a XMODEM update.
And finally, you WILL need to recompile your OpenAT application against the 7.52 firmware for it to run on the updated modems.
I will admit that I only use the DWLWin method to do modem firmware updates…
ciao, Dave