There used to be a Yaskawa VFD control program that could be used with a RS485 board to adjust the spindle rpm. I have one that I used to use before the setup that is being used now came out. I see that the file has been removed from ShopBot Labs though. I could look to see it I have it on an old computer.

Another thought, if you take the Spindle Control.exe file that is in the newer software (C:\Program Files (x86)\ShopBot\Virtual Tools\Spindle Control) and put it in your installed versions virtual tools folder and edit the virtual tools.ini file so it shows up on your tools menu, you might just be able to run it and change the RPM's by clicking in the rpm window and change the value, then hit enter and it should change on the VFD. You likely will not be able to control it from a file, but you could put a pause at the point of a tool change and manually enter the value and then continue. Worth a try. As a test, I can change mine manually this way without SB3 running. I just ran Spindle Control.exe from it's folder, entered a value, hit enter, and it worked fine.