Every time the printer is power cycled, it will attempt to spin the ribbon take up gear. If it is able to spin, it sets itself into Direct Thermal mode. If there is a ribbon present, it will be under tension from the print head, preventing the gear from spinning, and thus the printer will set itself to Thermal Transfer mode.

In most cases, this is all that is necessary, because the default option in the driver is to "Use Current Printer Setting", allowing the printer to control the setting.

The driver can override the setting if needed:
1. In Devices and Printers, right click the printer and go to Printer Properties > Advanced > Printing Defaults > Stock. Set Method to Direct Thermal or Thermal Transfer, as needed, then click OK to return to the Advanced tab.
2. Switch to General and click Preferences, then Stock, and make sure the same change is made.

Once both of those places reflect that change, the print job will force the printer into the specified mode.