As shown in season 5, where September(Donald)'s implant is removed, he regains hair, emotions, compassion. But if the the implant makes permanent changes to the brain as is told to Peter when he inserts the same implant into his brain, how does September's brain revert back to normal? Transcript:The human kind mentions this in Act 3 where Walter tells Peter that
"WALTER: Are you also aware that the changes will soon be permanent? And if that happens, there's nothing I can do to get you back."
My question is, If the change is permanent, then how does September transform to Donald?