opensuse dup

opensuse_upgrade.dup

Make sure that you modded the repos to use the releasever variable:
 # zypper lr
Repository priorities are without effect. All enabled repositories share the same priority.

# | Alias                             | Name                                                                                        | Enabled | GPG Check | Refresh
--+-----------------------------------+---------------------------------------------------------------------------------------------+---------+-----------+--------
1 | leap-${releasever}                | leap-${releasever}                                                                          | Yes     | (r ) Yes  | No
2 | leap-${releasever}-update         | leap-${releasever}-update                                                                   | Yes     | (r ) Yes  | No
3 | leap-${releasever}-update-non-oss | leap-${releasever}-update-non-oss                                                           | Yes     | (r ) Yes  | No
4 | repo-backports-debug-update       | Update repository with updates for openSUSE Leap debuginfo packages from openSUSE Backports | No      | ----      | ----
5 | repo-backports-update             | Update repository of openSUSE Backports                                                     | Yes     | ( p) Yes  | Yes
6 | repo-sle-debug-update             | Update repository with debuginfo for updates from SUSE Linux Enterprise 15                  | No      | ----      | ----
7 | repo-sle-update                   | Update repository with updates from SUSE Linux Enterprise 15                                | Yes     | ( p) Yes  | Yes

# cat /etc/os-release
NAME="openSUSE Leap"
VERSION="15.2"
...blabla...

Then you can override it and do a distupgrade - dup:
zypper --releasever=15.3 ref

zypper --releasever=15.3 dup --download-in-advance --allow-vendor-change