Is it safe to transplant hair into thinning areas, or do you risk damaging existing hair in these regions?

This is an excellent question and the answer ultimately depends on your doctor’s judgment and your personal hair and hair loss characteristics. The major concern, of course, is whether a transplant will damage the existing hair in the thinning area. If the existing hair is very limited, and likely to fall out eventually anyway, most surgeons would feel comfortable performing a transplant and implanting grafts into this region to restore some density. Even if the existing hair is lost, the net benefit of having it replaced with permanent hair will be worth it.
On the other hand, if the existing hair is relatively dense and/or in good condition, most surgeons would prefer that you first try medical therapy for 6 to 12 months, to see if you can slow your hair loss or even restore some hair. Some surgeons, however, may feel comfortable proceeding with a hair transplant. In this case, your doctor must be careful not to place an excessive number of grafts in this delicate region because this could lead to increased shock loss. Make sure your surgeon speaks honestly with you about the risk of shock loss and limited re-growth if you choose to go this route.
Don’t forget that everyone has only a limited supply of donor hair on their scalp. You do not want to jump into anything, particularly if you are not sure how your hair loss will progress in the future. Top hair transplant doctors will discourage you from rushing into anything and will help you create a long-term plan for combating your hair loss.