If you’d like to turn your BSP into a subtractive brush, simply look in the modes box. When you’re in Modes, please click on whichever brush you wish to use and then look at the bottom of the Modes box for ‘add’ and ‘subtract’. Simply switch it from ‘add’ and click on the bubble beside ‘subtract’. After you’ve done that, you’ll have the subtractive BSP like you originally wanted.
If you have any further questions, please let us know!