Retconning stuff in movies is a bit harder to do than in the comics, because in comics it's words and texts and images.
When you introduce organic webbing in the first film, how can the sequels introduce mechanical webbing all of a sudden?
Spider-Man 3 introduced a version of Gwen Stacy over Sam Raimi's objections (he didn't like the character) to present her as a potential rival for MJ but the entire Peter-MJ story of the first two movies was based on the idea that Peter only ever had eyes for her. So short of melodramatic contrivances and so on, you can't make her work. Then they conflate Gwen with Ann Weying and tie her to Brock.
Let's look at the one thing Spider-Man 3 retconned...they made it so that the Burglar who Peter let go was actually Flint Marko. So that meant that Peter wasn't responsible for Uncle Ben's death and his entire motivation and basis of characterization of the first two films is misplaced. It's baffling that this idea (which was Sam Raimi's contribution by the way) wasn't vetoed by somebody because it's completely dumb and it's an example of a retcon that's just not accepted. Nobody looks back to the first films and rewatch it thinking how Peter's confused and mistaken about his guilt because the movies were made with, and work with, the idea that his guilt over his Uncle's death is deserved and real. When you cast James Franco as Harry and develop an arc suited to Franco's strengths it's going to be hard to make Harry on-paper more like the character in comics.
Par 1 decisionmaking can be undone but it's not easy and it tends to create messes. It worked with MCU Thor and with WandaVision mostly because their first two parts weren't very good to start with and in the case of Wanda she had never been a protagonist so her backstory could be threaded through to introduce new stuff fairly easily.