But before I do it, I wanted to know all the steps for how to synchronise my data from my other devices to cloud.
The idea is that you'd just restart all your Opera's if they're open when you reset the passphrase. Then, you'd just log back into Sync in all of them and you'd be good to go.
And if it's surely that they will not be lost from my computer (even if it's open or even I do it later..) , not only the open tabs but also the history, the passwords, the bookmarks etc...
Absolutely no guarantees. Best to have backups of things first. You can export passwords and bookmarks. You can save your opened tabs as a speed dial folder before you export your bookmarks, and they'll be backed up with bookmarks. For history, you might be able to back up the "History" file in the profile folder (location is shown at the URL opera://about), but note that it might only work as a backup for Opera on your current user account on Windows. You can't export cookies, but you can back up the Cookies file in the "Network" folder in the profile folder.. But, that won't back up local storage and databases for sites. You can't really export settings either. But, you an back up your "Preferences" file. You can't really back up your extensions either.