Is Upgrade to 4.x safe for me?

Hello everybody,

I am considering an upgrade of my omv 3.0.96 to omv 4, Debian 8 to Debian 9.

I´ve got the following plugins installed:

• ClamAV
• Backup
• Apple Filing
• Docker (CE-Repo)
• Fail2Ban
• LetsEncrypt
• MySQL
  
  
  • Debian 9 has MariaDB as default. Will the upgrade result in a smooth transition from MySQL to MariaDB?
• Nginx
• RSnapshot
• FlashMemory

I believe the Fail2Ban plugin hasn't been ported to omv4 yet, so I'll have to uninstall it. LetsEncrypt, MySQL, Nginx and FlashMemory are really important for me.
Emby is installed via custom repository. Do I have to change the repo from Debian8 to Debian9 before or after upgrading omv?

Will it be safe for me to upgrade?

And another very important question: How do I upgrade? Simply execute "omv-release-upgrade"? Will this command also upgrade the OS from jessie to stretch? Do I first have to change sources from jessie to stretch and Upgrade the OS?

Thank you all for you help. Much appreciated.

Thanks for your answer.
But the big question remains: how safe will the upgrade procedure be, and how exactly is it done?

Thanks...
I will go and make a backup with clonezilla. Lets hope the Upgrade will go fine.
Will let you know.

Update:
I did the recommended backup via clonezilla --> worked very well including testing of the image
Did the upgrade --> worked well
First thing I encountered was the MySQL Database could not be accessed. After resetting the root/omvadmin password this worked as well. Actually, I don´t really know what happened exactly or what made it work in the end.
I also had the "rrdcached plugin could not connect to..." error in syslog. After enabling and disabling the Monitoring this error disappeared.

So far, I'm really happy with the upgrade.