Is it possible to experience swelling years after a hair transplant?

I don't believe this is related to swelling; it might be diffuse fibrosis in the deeper layers of the skin. A scalp biopsy would likely provide the accurate diagnosis. Massaging with topical cortisone creams and/or applying silicone gel may help alleviate some of the fibrosis. If these measures prove ineffective, steroid injections could be considered as an option.