No problem. When I started to use my first Raspberry Pi (2), I did the same mistake and was wondering why the kernel version was always lowered when it got upgraded via apt upgrade. So I just wanted to make the point clear here, that it is not an end user tool.
What do you want to reverse? It is not easily possible to revert the firmware migration, as it requires to undo the changed partitioning as well. And it would break usage on RPi 5 as fast as there is another kernel upgrade (as this cannot be applied without the partitioning change). So as long as you have no strong reason, leave it as is now. Earlier or later we will roll out the migration to all users via dietpi-update.
Can I now create a backup in the dietpi settings with the system migrated to raspberry pi5? I ask because Jellyfin I have a lot of work done. If the answer is YES, I can use this backup when the clean installable version of Raspberry Pi5 comes out.
And finally, running rpi-update, what does it modify? the operating system or modify the rasberrypi? bios or firmware chip.
By deleting the SD and starting from 0, have I resolved my error of running the rpi-update?
Nothing you need to do on a fresh install. Just stay away from rpi-update if not explicitly ask by a Raspberry developer.
no need to start a fresh install once we released an own image for RPI 5. There will be no difference and there is no downside to continue with your current setup.
. So I just wanted to make the point clear here, that it is not an end user tool.