COM port assignments changing for DLP USB232M
It's not a hardware issue. Windows arbitrarily assigns COM port numbers whenever an FTDI FT232 part is connected, regardless of whether or not it's previously seen that particular individual device.

If your software needs to be able to reliably identify which COM port is currently assigned to a particular board, you can use the FT_Prog utility (http://www.ftdichip.com/Support/Utilities.htm) to assign a distinct serial number string to that device -- this string will be stored in the SPI serial EEPROM attached to the FT232. 
Then when you open the device, provide the FT_OPEN_BY_SERIAL_NUMBER option to the FT_W32_CreateFile function. See D2XX_Programmer's Guide(FT_000071):
http://www.ftdichip.com/Support/Documents/ProgramGuides/D2XX_Programmer%27s_Guide(FT_000071).pdf