That’s a great question! In the end, it’s important to choose whatever means necessary to achieve your desired goals for size, but more importantly shape! My patients generally seek to have a perky or curvaceous butt that is shapely, whether they are full-figured and voluptuous or shredded fitness types, and everything in the middle. Depending on your end goals, you may either need multiple grafting procedures and not necessarily because that previous BBL went away. Consider a consultation with a plastic surgeon that does all 3 major types of enhancements (Implants, Fat Grafting, and Combinations) to understand all your options.
No it is not necessary to go two rounds of surgery to get a nice result with a BBL. Some patients choose to do a second round if they gain weight and want more liposuction. Others go a second round because they weren't happy with the results of the first surgery. Some patients are obese and require multiple rounds of liposuction to get the tiny waist that is popular with a BBL. For example here in TX we can only take 5000cc of lipoaspirate in one setting without the patient staying overnight with monitoring by a nurse. Some patients don't want to do this and choose to go two rounds of surgery without staying the night. The butt does go down with time (~30-50% of the volume that was injected is lost over the first 3-6 months after surgery). Enough fat should be injected that even after the loss, you will still have a obvious volume increase and shape change. Make sure you go to a board certified plastic surgeon that does a lot of this type of surgery to make sure that you have the best change of success in one surgery.