Hi,
JFI: We experienced in 2 instances, that coming from CMK 2.3 that notification-rules weren’t properly converted (the notification-plugin-params got lost, messages from cmk-update-config for that appeared in the update.log ).
We contacted support, which could reproduce the problem and informed the Devs to fix.
So probably best to keep a copy of your notifications.mk or to make some screenshots before doing an upgrade/update, currently 
Micha
The update creates a backup by itself from the original file.
Was this not existing on your system?
It was present, but I believe that if cmk-update-config is executed manually again — for example, during troubleshooting — the backup file might get overwritten. I’m not entirely sure, though.
The old notifications.mk should still contain the old rules.
Regarding the update problem we are currently implementing an update step that migrates an 1.6 HTML Email “From” parameter to the correct values.
This was ‘{‘from’: ‘’myfrom@address.com”}’ and is ‘{‘from’: {‘address’: “myfrom@address.com”}}’ since 2.0
(the rule already looked broken on opening them in the GUI since 2.0)
Not sure if it’s the problem in your case but that’s the only problem we have seen so far.
will be fixed with Werk #18859: HTML email: Migrate old 'From' format
note:
If you are already on 2.4, the migration of the notification parameter is already done and the configuration does not contain such old format (update action is skipped)
With my last upgrade i found one “work around” if you have not too many rules in your system.
Open every rule on 2.3 and save it again without doing anything. The result was an upgrade of all notification rules without problem. 