Conversely, I often find coding agents privileging the existing code when they could do a much better job if they changed it to suit the new requirement.

I guess it comes down to how ossified you want your existing code to be.

If it's a big production application that's been running for decades then you probably want the minimum possible change.

If you're just experimenting with stuff and the project didn't exist at all 3 days ago then you want the agent to make it better rather than leave it alone.

Probably they just need to learn to calibrate themselves better to the project context.