I am sorry if this is too late in the game to be of any possible assistance... These comments probably are completely off base. If that is the case, I should move them to where they belong.
Upgrading From Stretch to Buster
Not 100% different from upgrading from Jesse to Stretch.
I say DON'T DO IT! Yes, move your stuff from Stretch to Buster, but do not risk your Stretch on an upgrade is my gut reaction.
You might already know this next part.. but in case someone else reading this thread does not....
If you have not had experience with virtual machines for running linux, then, I will say this. You are missing out on something as big as the whole rest of the linux world. That will sound like exaggeration if you have not found out what virtual machines are. When you find out, you will see why the emphasis I am putting on virtual machines is appropriate. If everyone already knows all this, then I am sorry to waste your time mentioning it.
I admit my situation is unique, because I am extremely stupid and lazy. With that as my starting point, it is a billion times easier to just make a brand new virtual server with Buster then copy what is needed from Stretch to Buster, without putting stretch at risk of problems due to the upgrade.
There is a special app that makes this run smoothly. It is called AreSink, spelled rsync. I think it is $29.95 from Amazon but if you know where to look, it is a lot cheaper from Debians (I think Debians charges one "apt install" command per instance of Debian, and that is it). The only bad thing about AreSink is getting used to remembering to forget about ftp and other ways of copying files. rsync -av pulling from the old machine into the new one never breaks the old machine and the new machine is under construction so foul ups are no big deal. Accepting this is not for everyone, especially those unfortunate souls who enjoy agony.
On my computer at home, I sprung the $100 for VMWare, so I can have all the instances of Linux I want. I found used 8TB disks on ebay plus some 500G PCI SSDs to run what I need going fast. Put 64G of RAM on board, and off you go, no worries for years. It is like my computer is a data center.
I never upgrade a server with a major upgrade. Instead, I make a new server with the newer version of the OS, then copy everything from the older verstion to the newer one. By not applying the upgrade, the old server is preserved. It never loses its ability to do what it has been doing. After awhile, all the stuff is setup on the new system and I just leave the old one not running. Finally the day comes when I just delete it
Upgrades within a major release have never been a problem. Just use apt update;apt upgrade; This is one of the beauties of Debian. Sticking with apt is a good way to avoid problems.
The same strategy can be used with cloud servers.
Just say no to major upgrades. Once you give VMWare their $100, all servers are free, except Windows, which is a bonus reason to avoid windows where possible. Build a new Linux machine as a virtual machine, and run it, and keep your old one safe in the process.
Servers in the cloud are more costly,but still cheap enough to just make a one for upgrades and never apply the upgrade to the one that has been running awhile. Not need to risk that. When everything is moved to the new server, just turn the old one off and the costs go back down to what you had before the upgrade.