upgrading to 15.3 use zypper dup --allow-vendor-change or --force-resolution or none??

I’m upgrading from 15.0 to 15.1 to 15.2 to 15.3, I opted for the online upgrade following the instructions in:
https://it.opensuse.org/SDB:Aggiornamento_del_sistema_ad_una_nuova_versione
that should be the italian version of
https://en.opensuse.org/SDB:System_upgrade
in the italian version I read this:
“Tenere presente che openSUSE Leap 15.3 ( ‘attualmente in alfa e pianificato per il rilascio finale a luglio del 2021’ ) utilizza i binari di SUSE Linux Enterprise, quindi l’aggiornamento da 15.2 a 15.3 comporta un cambiamento del fornitore da openSUSE a SUSE LLC. Consigliamo quindi agli utenti di utilizzare” [take into accont that Leap 15.3 (currently in alpha…) use binary of SUSE Linux Enterprise, so the upgrade from 15.2 to 15.3 has a change in the vendor from openSUSE to SUSE LLC so we suggest to user to use]

# zypper --releasever=15.3 dup --allow-vendor-change

“o in seguito al quale era necessario solo il cambio del fornitore.” [or afterwards at that was necessary only the change of vendor]

# zypper --releasever=15.3 dup --no-allow-vendor-change --force-resolution

but in the english version this section is not present
"9. Add extra repositoriesSearch for updated openSUSE Leap 15.3 compatible third-party repositories that you used before — if you still need them — and add them.
Warning: Use with caution. Using third-party repositories may break your system or cause instabilities.
zypper addrepo --name <name> <url> <alias>
Or, if you have URL of a .repo file:

zypper ar <url.repo>

Executing zypper up may be enough to update your software from these extra repositories."

which command I have to use to correctly upgrade from 15.2 to 15.3?

As stated on the Italian page:

*[take into accont that Leap 15.3 (currently in alpha…)[/i]

…so I guess the instructions are obsolete since it describes the upgrade process to Leap 15.3 Alpha version. When upgrading to the version 15.3 stable, no –vendor-change command option is required:

The libzypp package version 12.25.8 introduced whitelist for the SUSE LLC and openSUSE vendor exchange to allow seamless migration. This whitelist removes the need to specify --allow-vendor-change for openSUSE and SUSE LLC vendor exchange only.
More on this in the release notes: Release Notes | openSUSE Leap 15.3. Also, you can check the whitelist on the upgraded system:

david@atronach-opensuse:~> cat /etc/zypp/vendors.d/00-openSUSE.conf [main]
vendors=openSUSE,SUSE,SUSE LLC <https://www.suse.com/>

I don’t think that’s necessary. There has been a change in zypper, so that it now recognizes several vendors as equivalent. But it is probably important that your Leap 15.2 is fully updated, so that it has the newer “zypper”.

The core command in the upgrade process is:

# zypper --releasever 15.3 dup

No –force-resolution command option is required, –no-allow-vendor-change cmd option is redundant since the package manager adheres to it by default unless you’ve changed the solver.dupAllowVendorChange directive in /etc/zypp/zypp.conf. Regarding –allow-vendor-change, it’s unnecessary in my opinion but is suggested according to the Start Up guide at Start-Up | openSUSE Leap 15.5.

There’s more to the upgrade process though than a just single command can cover so read up the English “SDB:System upgrade” page which I thing you should follow instead of the Italian version which is probably obsolete. And look at the upgrade instructions in the Start Up guide too.

Both guides are missing one thing however: Running rpmconfigcheck after the zypper distribution upgrade command which will list all *.rpmorig, *.rpmsave and *.rpmnew files which might be good to get merged with their active configuration files counterparts. You can merge them with some diff editor like vimdiff. But then again, this is just a bonus to do the upgrade process proper, maybe it’s not necessary but I did it after Leap 15.2->15.3 upgrade just to be on the safe side.

many thanks, do you think is a good idea to add before the zypper dup to 15.3 these two repositories thet in 15.2 there wasnt?
http://download.opensuse.org/update/leap/15.3/backports/
http://download.opensuse.org/update/leap/15.3/sle/
or the process of upgrade do it by itself?

Those repositories are added by post-GA updated 15.3 openSUSE-release package. But this means you probably need to run update immediately after dup to fetch updated packages from these repositories.

I just ask for completeness.
You are making update steps from a rather old version, Did you check if all repository definitions have the $releasever in the proper place? Else the --releasever option will not function.

I changed the 15.0 to 15.1 in the repositories address manually and then used

zypper dup --download-in-advance

and the same for 15.1 to 15.2 and for 15.2 to 15.3 and for 15.3 I added
http://download.opensuse.org/update/…5.3/backports/
http://download.opensuse.org/update/leap/15.3/sle/
and luckily everything went flawlessly until now, I have only to try if scanners and wireless connections and printers not present here does works.
I chosen online upgrade becouse I have my router under UPS and the pc was my son’s laptop that in case of lacking of electricity has a battery, and olso the --download-in-advance option helps