First off:>Please show me the point where I said that the characters never changed on purpose.>None of them had canonical reasons to act different
Secondly, don't use falsehoods such as Lucy supposedly being based on Schulz's wife (even if, for whatever reason, you don't believe it yourself...?). By doing so you perpetuate myth. On a substance level, its basically equal to the "theory" that Peppermint Patty and Marcie were secretly written to be lesbians.
Lastly and most importantly, what you're pushing back against is, at its very basic level, the practice of literary analysis and the verifiable fact that Schulz's comics had depth not spoken directly. There are many examples of further depth in Peanuts.
For the purpose of explanation I'll use a very popular concept often mentioned in these threads: Peppermint Patty's veritably strong relationship with her father and her subsequent love for Charlie Brown. There are many specific strips that while obviously not directly saying as much, imply an acute mental association between Peppermint Patty's father and her love for Charlie Brown. She will mention her father to Charlie constantly, these interactions (often posted here) make an association between the two men in her life blindingly obvious. Just a couple examples of this are the flowers Patty receives from her dad that she presents to Charlie Brown (October 4th 1970) and Patty informing Charlie Brown of her father-given nickname as a "rare-gem." When Charlie agrees with this moniker she's very happy about it and falls deeper in love with him (October 4th 1971).
If you want a direct example of Schulz making this mental connection Peppermint Patty has between her father and Chuck so abundantly clear that a reader could come to no other conclusion, just look at pic related (June 20, 1976).