Well, it depends on the X really. The first scenario, the X could be anything. If for the 2nd scenario, X is 15 for example. Then it would be different, since that would be Scenario 1's reincarnator would be living their life normally up until X, assumming that that isn't 0 of course. If it was a villainess, spoilt and all, this would mean that they actually have the spoilt personality of the said villainess. If they change when they get their past life memories, they would do so because to previous experience when not very rich like now or whatever else they have learnt. In contrary, the 2nd scenario reincarnator would have no relations to the current body they are in. But if X for both scenarios were 0 (years old). They are basically the same.
Sorry for not being expecific, i meant what you thought (〜￣△￣)〜
No worries! Totally agree ヾ(❀╹◡╹)ﾉ~
Do you think that the "i lived in this body till i was X years old and suddenly remembered my past life in which i've read a novel about this life (aka i know the future)" and the "i reincarnated in the body of X character when they were X years old with the memories of my past life in which i've read a novel about this life (aka i know the future) and received their memories (of all those X years)" tropes are different or essentially the same? Both plowise and putting youself in that person's place (One you've lived in the body and the other you just got their memories).