Last night, I did my first 23.x.x to 24.x.x upgrade. I didn't find a proper how-to till after I ran the command, so I wanted to double-check that I didn't mess something up. I used wget to download the image to /root and ran sysupgrade -n from there. I didn't run into any errors, but it seems like best practice to download to /tmp.
Is there problem running the command from /root?
You can use auc utility (owut in later versions) or unified luci-app-attendedsysupgrade web interface and preserve also package list, not only config files.
If root has sufficient space there is no problem, just that flash is worn off doing so when you could use ram disk /tmp.
Not really, but... sysupgrade unmounts the partition in which /root exists in order to overwrite it with the new image, so the image must be elsewhere (in this case, /tmp, which is a ramdisk on the standard installs).
You probably saw that sysupgrade immediately copied your image to /tmp, so you can save that step by simply storing it there, but no harm comes if you don't.