There are a few options available out there for you currently. The first question I would pose for you would be: What are your goals? Would you prefer to have more hair on top (i.e. additional hair transplant(s)) or would you rather just have a natural appearing balding scalp. If you think that you would like to have the latter, the grafts can then all be removed. This process may take more than one session, as sometimes a few hairs continue to grow after the first pass. Still though, after the initial pass and after the areas have been healed, you will notice an immediate improvement. The downside of this strategy is that you may still have some noticeable scarring in those areas. Depending on how you heal normally, these scars may be minimal. Laser resurfacing can also be done as a measure after the grafts are removed to also help with the irregularities of the scalp and the scars that are created.
The other route to take, would be to continue with hair transplantation in the frontal half of you scalp. It does appear that you have adequate donor from the photos of the back and sides of your head. A repair session would most likely require initially two steps. It appears that when had your first surgery the grafts were placed very low in the corners of your hairline. At your age now and taking into account the amount of hair loss that you currently have, I would recommend bringing those corners back a little to make them appear a little more natural. The first step would then be to remove those grafts and to "recycle" them into an area on the top of your head. At the same time, I would probably break up and reuse a few of the other grafts in the very front row or two of large grafts just to create a softer hairline. Finally, and about 3 months later, I would recommend a full hair transplant session to cover your existing grafts and to recreate a new and softer hairline. I would maximize that surgery by doing 2500 -3000 grafts if possible. I would recommend the "strip" (FUT) method, as with this we would be able to get a large number of grafts and you already have graft scars on the back of the scalp from the prior surgery.
12-18 months later you can then decide if you desire an additional session to go farther into the crown and / or increase the density in the frontal region.
I would recommend looking into experienced local doctors and asking if they work much with repair cases. Most experienced docs will also have before and after pics of cases.
I hope this helps.