Can I reverse a hair transplant (3500 stem cell FUE using tool 0.6 to 0.8 and literal slit in recipient area) since 2.5 years?

You cannot reverse a bad hair transplant surgery.  A bad hair transplant surgery is difficult to fix.  Please consult with a doctor for your particular case.

Dear houssam23,
Thank you for posting this question. The short answer is no, you cannot reverse a hair transplant. I suppose technically you could remove all 3500 follicles, but that will create an additional 3500 scars on your scalp. This is in addition to the 3500+ scars that are already on your head as a result of the FUE procedure. I can assure you that having 7000 scars on your scalp is not something that will easily cover up with SMP or with any method other than wearing a wig. There’s an old saying in hair restoration, and it goes like this: the good thing about the results of a hair restoration procedure, and that is they’re permanent. The bad thing about the results of a hair restoration procedure is that they’re permanent. Buyer beware has been the rule, and has never been more applicable than today, where I am seeing a dramatic increase in the number of surgeons who are acquiring an FUE device, holding themselves out to the public suddenly as hair restoration experts, and trying their hands on a new procedure on unsuspecting patients. For the past 2 years the number of very poorly-performed hair restoration surgery results I have seen at my center has risen dramatically. It is unfortunate that so many are getting hurt and cosmetically disfigured as a result of these practices. I hope this information is helpful to you.
Kind Regards,
Ken Anderson, MD, FISHRS

I am sorry to hear that you have some type of complication following hair restoration surgery. I really don't know what you mean with: FUE stem cell hair transplant. Adipose Derived Stem Cell Therapy from autologous fat to obtain SVF (Stromal Vascular Fraction) that includes different type of cells such as endothelial, hematopoyetic and stem cells, etc. The entire must be done properly, the process includes: lipoaspiration, wash and digest the fat then apply  fat and SVF in the hair loss/recipient area. We did an study about SVF in hair loss patients that showed around 20% of hair regeneration. I really don't know which type of FUE stem cell transplant was performed. 
It is a great question to get back where you were before starting your hair restoration. In theory could be possible to reverse your case but the reality is totally the opposite because the scarring and hair growth and survival and I will suggest that is not a good idea to do it. However, it is important to evaluate your scalp completely then to find the best treatment options. I will suggest to look for a 2nd or 3er opinion in your state or country. It will help if you can send global pictures and then some close-ups (zoom) pictures too, please include your donor area. The smartphones have great zoom features. Good luck.