I'm just talking about my experience, and what I experience is similar to you in that I thought that he had been "replaced" by his addiction and that for most of the relationship, he was fine. Truth is, he was never fine. He was gradually getting more and more addicted, sicker and sicker, but he had been trying hard to hide it and act "normal". The worse his addiction became, the less he was able to hide it. I think it was like there was two of him: the him that wanted a normal life just like everyone else, and since I was willing to be there for him, he wanted me in it; and the him that loved drugs/alcohol over anything else on earth, even his family, his friends, or his wife and pets. The addict in my life was and still is a deeply troubled person in denial of their pain... so they drug or drink it away. It can seem like a sudden change, simply the addict was putting on a show for you -- mine kept it on for a decade. There were times when the mask would slip and I would say, "but hold on..." and he would intercept my gut feelings with some explanation that seemed only slightly implausible, but because I loved him and I wanted to trust him, I did trust him. I don't know if that answers your question or not.
I think that I used to think that he was "replaced" by a walking talking disease, but the truth is that the disease (if you think addiction is a disease, which some people don't), was always part of him. It takes a lot of work to get drugs or alcohol to abuse, a lot of planning. In my ex's case, he had to first buy the drugs, then he had to buy a thing he was going to use as a method of delivery, that's two separate shops and/or dealer already. After that, there's making the bong (because he spends all his money on drugs, he didn't have a functioning bong, he had to make one), then there's using scissors to cut the drugs into little bits and to mix it with some other rubbish he enjoyed smoking. And also, there were probably other drugs... because the side effects really seemed like there were other drugs. Then there was the "having to sneak it all past the wife" and also "coming up with a story to tell the wife", and also gargling with mouthwash / bathing afterwards (and coming up with an excuse for that too... and that was in the early stages, when he still had control over his use and he was not so lost that he stopped bathing entirely because it took time away from his drug use). At ANY point during this lengthy process, he could have said to himself, "what the hell am I doing?" He chose to start. He chose to continue. So I think that there was something in him that thought it was okay to keep doing this until he was totally out of control, until he lost his family, his home, his money, his health. He started the drug use because he felt that, for him, there were exceptions to the rule of trust being integral to a loving relationship.
So for me, I would say that there was always a part of the addict that is addicted, and likely always will be. I don't know if that answer helps you or not.