When I watch Friends as reruns, I can tell in the very early episodes, they act differently. Somewhat like caricatures. I've wondered if it was just my interpretation or if they did act differently then compared to say season 5. I know at the end, esp. the very end, they were too sentimental. Is it just my thinking, or was there a change?

There definitely was a change.

During the first episodes, they hadn't yet found all their voices, yet. The characters were still evolving, and being shaped by the scripts. Early on in the season, Monica hadn't yet become a neat freak.

I actually prefer this over the final seasons, where they were all caricatures of their former selves. This was most noticeable with Ross and Monica.