I feel like Ian won't change. He will maintain his mannerisms as he was introduced. We just don't have enough information to ~how~ the memories are remembered. like, is it similar to that feeling of how we random cringe at things that happened eons ago at the most inconvenient times type of memory? or is it like a detailed log we can clearly recall information to? or maybe something in between.
I'm still sold on the fact that Sienna, Sol's Prime Minister, is the reincarnation of the freckly guy that is with Sol and Brian all the time now. He seems like he is "Sienna" because of how he acts. Like I can tell the difference between the two. I can only make assumptions at ~his~ inherited memory. It doesn't seem like it affects him in the way we fear it might affect Ian. Sienna is our closest test dummy to dissect.
Okay, your theory is quite interesting. But what if...what if he turns out to be Russell?
Considering that Ian and Russell is quite close, similar to the closeness between Brian and that freckly guy. Then the fact that Brian's reincarnation was born in Derwin, there's a possibility that the freckly guy's reincarnation may not be in Archaia too.
After thinking for a moment, i have several theories about these:
1. Whether Ian would get his past memories as Brian or not?
2. Or if he gets his memories, would he still be himself, or would his personality change to be exactly like Brian?
In my opinion, the fact that Ian is Brian's reincarnation will not change, and we have to accept that. Ian and Brian have the same essence and "core", so it's not surprising that they have similar traits, this is what readers often miss. Some of the most prominent are their curiosity and the way they sneak out of whatever confinement they're placing.
But again, even though the core is the same, different experiences and upbringing can produce different characteristics, It's called nature vs nurture. Human personalities won't change that easily, especially since Ian is already an adult. So I think even if Ian got his past memories(as Brian), he wouldn't change much. I'm sure there would be a couple of things that change, but Ian would still be himself. What's different is that he remembers his past memories.
Honestly, as a reader who is biased towards Ian, my enthusiasm for reading this season is not as high as the previous season. But because Brian is still Ian's past, I can't help but like him too. Maybe, the most tragic character in this story is Brian. Ian's journey is still long and unpredictable, there's even a possibility that he can find his own happiness and live longer than Brian. However, Brian's story just ended there, on the day he was killed.