You con only read and write at the byte, word or Dword level when you use Profibus communications.
If you want to have a single bit modified, then you have to wrtie the new value of the whole word back to the Profibus Control word address.
This is basically what you do when you start the drive. (I am not sure if the values I am using as an example are the same as the ones you use, these are from memory and are based on MM4 experience). To start up, the drive must see a rising-edge on the LSB of the Control word (change the value of the LSB from 0 to 1). So you start by writing w#16#447E to the Control word addrees (therefore setting the LSB to 0) and then you write w#16#447F to the same address, setting the LSB: the drive will now start up and run at the Speed reference that you have set.
Hope this helps,