[HOWTO] update from 8.0 Flidas to 11.0 Aramo

Skipping versions on upgrades is not supported; only to the next version. Your choice is to upgrade from 8 Flidas to 9 Etiona. Then upgrade from 9 Etiona to 10 Nabia. Then upgrade from 10 Nabia to 11 Aramo. Or you can reinstall and go straight to 11 that way, and then avoid having to do so many in-between upgrades. But a direct upgrade from 8 Flidas to 11 Aramo is not supported path.

Installing from the ISO would certainly be faster here. Even more if you know how to save and restore your list of installed packages. Here is a way: https://trisquel.info/en/wiki/cloning-system-or-how-make-copy-installed-packages-one-computer-another

(I believe Synaptic will ignore the packages that ceased to exist under the same names, a minority.)

If your /home is on a separate partition (you got that if you chose Trisquel 8's default installation), you can reuse it without formatting. It is a box to uncheck in the "Something else" type of installation, from the live system. Doing so, you then want to create the same first user (with id 1000) with the same login. id 1000 executed in Trisquel 8's terminal will tell you which one it is. For instance, here, it is "banana":
$ id 1000
uid=1000(banana) gid=1000(banana) groupes=1000(banana),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),117(lpadmin),127(sambashare)
If there are more users, you can recreate those from the newly installed Trisquel 11, still taking care of having the same ids as now. That command will show those UIDs:
$ stat /home/*

Anyway, you want to backup the user files before installing Trisquel 11!