People can definitely change over the course of decades, however - much of that change is internal, stuff like personal philosophy, ways of thinking etc. When it comes to external change - how others see you, things like mannerisms and affect are below consciousness and don't change a whole lot over the years and if they do it's often in very small and subtle ways. When those things do change, it's often due to really concerted effort, like say when you focus intensely on trying to become more confident and present yourself in ways that are atypical for how you've been perceived before. Even when people do feel they change a lot, it can often be difficult for others to see because much of that change is going on in how people witness themselves over the years. I think this is where Picard can feel a little off now and it probably would've been of benefit for Stewart to re-engage with his TNG performances as he was preparing for this role and study that part of himself.
I don't think the change being depicted in Picard here is very nuanced, basically. It's using a time skip which is a shortcut and I think despite Patrick Stewart being a great actor there's something missing in integrating his past into who he is now in the more subtle performative ways. It's sort of a passive way of invoking change, to rely on 'time passed'. The sense I get from the writing so far, is that it's weak in the psychological and philosophical areas while pursuing a traditional political arc. It's only a few episodes in though, so I don't want to be close minded and say that they can't expand on these areas in interesting ways.
