Is it possible to upgrade from Ubuntu to Lubuntu without losing everything? (Files, Accounts, Data, Document, etc.)
backups are worthwhile - so always backup first.
yep; but still better to backup should anything go wrong.
another alternative is to add LXDE to your existing ubuntu system; eg. my "ubuntu 16.04" system also has
lubuntu-desktoppackage installed; so I can at login have unity (standard ubuntu), xfce (xubuntu), lxde (lubuntu), mate (ubuntu-mate).
note: loading many desktops is not intended. loading many desktops like I do can cause them to misbehave, or your system have more quirks than a pure system, but it's an option. they will all be 16.04; so if its the later 17.04 you want; you'll need to re-install (remembering backups first are a good safety measure!!)