What if you decide to get a FUE hair transplant very early when the crown hasn't shedded all the hair?

You should not have preventative hair transplants if you have a full head of hair.  You probably need an examination and the doctor needs to understand your goals and expectations.  

Although it's not possible to have a hair transplant in the very early stages (before significant thinning has occurred), it is certainly possible to have a transplant when the hairs are still present but very thin. Usually this is a stage where you can see through the the skin of the scalp is visible. All in all, if you have concerns about your hair, see a hair specialist. Medical options (non surgical) may be advised first, including finasteride, minoxidil, laser and PRP.

Most reputable hair transplant surgeons do not do hair transplant in the early stages of hair loss in the crown area. It is best to try a medical regiment such as using minoxidil or propecia. 

This would be malpractice for a doctor to perform preventive hair transplants because it could damage any reasonable long term plan for your future. Use the drug finasteride as in males, it works well in the crown.