So, I've spent a few days working on this problem.
WARNING
First thing I eventually realised was that the update from Sonoma to Sequoia "lost" a large proportion of my Keychain Access Secure Notes, over 1/3 !!!
I had a feeling that some Secure Notes were missing and others out of date. Then I noticed that a Secure Note I know I had used a few weeks ago was no longer present. Dug around in various backups and snapshots and found a login keychain which was dated just before I updated. This had 519 Secure Notes but after the update, there were only 322!! The update to Sequioa had "lost" 197 Secure Notes - no idea where they went, certainly not in the new Passwords app !!!
Miln Keysafe tool
If you were a big user of Keychain Access Secure Notes, as I was, Apple's current copy/paste solution to the Notes app isn't a realistic option.
I eventually was pointed to the Miln Keysafe tool. Not ideal for many users as it runs under Terminal and only does half the job. It does extract the Keychain Secure Notes to text files but it is then necessary to use Automator/Applescript to add the file name to the first line of the txt file before importing into Notes. I haven't yet managed to get that righ … yet.