I decided to work an old game from v2.72 and converted to the last version of AGS.
Even if ags still recognize global messages from previous versions, I can't find global messages from 900 to 999.
From what I understand, newer editors do not provide any means to edit the Global messages.
The relevant article in the manual:
https://adventuregamestudio.github.io/ags-manual/UpgradeTo30.html
QuoteGlobal Messages
Global Messages are no longer supported and should be considered obsolete -- there's really no need for them now that the interaction editor has gone. Any global messages that you had will be retained and will still work, however the AGS 3 editor provides no way to edit them.
So, they simply stay in data, and may be edited by hand in Game.agf (if you're careful, but it is simply an XML). They may also be completely removed using "File -> Remove Global messages" command. Unfortunately, Editor also does not let automatically copy these messages over to some supported form, like into a script, or Global Variables, which is a shame, and clearly an oversight in the upgrade process.
Strangely, Room Messages can still be accessed (from the room properties), and seem to retain all the old options.
EDIT: I opened a ticket regarding this (https://github.com/adventuregamestudio/ags/issues/1854), but it's probably going to be low priority for the dev team.
How strange the manual doesn't say global message is obsolete.
(https://content.invisioncic.com/r322239/monthly_2022_12/large.display-message.png.56b2529ae5c538c1b7b299c6da6dee93.png)
Anyway. I managed to solve the problem.
Just open 2 AGS editors the old and the new.
Copy and paste the lines to new AGS by using Display instead DisplayMessage.