Upgrading from Owncloud 9.1.1-1.2 deb packaged to Nextcloud 9.x not packaged?

Hi Friends,
I’ve installed on my Debian Jessie VPS Owncloud 9.1.1-1.2 and its dependencies:

owncloud-deps-php5             9.1.1-1.2
owncloud-files                 9.1.1-1.2

Managing this kind of installation, debian based, I’ve seen on the time, that it’s a little bit more complicated upgrade a packaged OC than a simple pure file installation. I think will be the same for NC, so I would ask you if exist a tutorial for move from a packaged installation to a pure file installation.
considering that I’ve some user and data on the running OC:

is there a way to upgrade from an OC deb version to a pure NC file version keeping all the data?

Many many thanks