What cup size would I be if I had 500 or 750 grams removed from 40DDD breasts?

Difficult to say without an image. Likely a DD or D.  I recommend an in-office examination as well as a detailed discussion with a surgeon who you are comfortable with.   Finally, make sure your surgeon is a Double-Board Certified Plastic Surgeon certified by the American Board of Plastic Surgery (ABPS), a member of the American Society of Plastic Surgery (ASPS), a member of the American Society of Aesthetic Plastic Surgery (ASAPS), and a fellow of the American College of Surgeons (FACS).Best,Dr. DesaiBeverly Hills Institute for Aesthetic Plastic SurgeryHarvard Educated, Beverly Hills & Miami Beach TrainedDouble Board-Certified Beverly Hills Plastic Surgeon

Removal of 500 g from each breast with a DDD cup may reduce the individual to a D or so.  This is greatly dependent on density, which changes with the amount of fat in the breast and the age of the patient.Kenneth Hughes, MD, Board Certified Plastic SurgeonLos Angeles, CA

Thank you for the question and congratulations on your decision to proceed with breast reduction surgery; this operation tends to be one of the most patient pleasing operations we perform.
 
There is no direct correlation between the amount of tissue removed and the ultimate cup size that a patient will wear after breast reduction surgery. There is simply too much variability among bra cup manufactures to make it a useful metric when it comes to preoperative communication and/or post operative predictions.
 Before undergoing the breast reduction procedure it will be very important to communicate your size goals with your surgeon (who you should choose very carefully). Most patients wish to achieve enough of a reduction to help with their symptoms while remaining proportionate with the remainder of their torso. 
With the goal of improving communication with my patients I find the use of photographs of “goal” pictures (and breasts that are too big or too small) very helpful. For example, I have found that the use of words such as “natural” or “B or C cup” means different things to different people and therefore prove unhelpful. Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup size may also be inaccurate. 
Once you have communicated your goals carefully, your plastic surgeon will be able to provide you with an estimation of how much tissue will need to be removed, in order to achieve your desired outcome.
Best wishes with your breast reduction procedure; hopefully you will be very pleased with the outcome of the procedure performed.