"Support for upgrades and downgrades that span more than three Junos OS releases at a time is not provided, except for releases that are designated as Extended End-of-Life (EEOL) releases. EEOL releases provide direct upgrade and downgrade paths—you can upgrade directly from one EEOL release to the next EEOL release even though EEOL releases generally occur in increments beyond three releases.
You can upgrade or downgrade to the EEOL release that occurs directly before or after the currently installed EEOL release, or to two EEOL releases before or after."
Is the minor Version relevant in this context, e.g. is 18.1R3-S9 the same as 18.1R1 in regards to upgrade compatibility? For example, can I upgrade from 17.3RX to 18.1RY or 18.2RZ (X,Y and Z being random minor Versions) ?
> Is the minor Version relevant in this context, e.g. is 18.1R3-S9 the same as 18.1R1 in regards to upgrade compatibility?
Only the major version is relevant, e.g. 18.1, 18.2, 18.3. You can ignore any R or S parts behind these releases in terms of upgrade considerations.
> For example, can I upgrade from 17.3RX to 18.1RY or 18.2RZ (X,Y and Z being random minor Versions) ?
Since Junos 15.1, all Junos releases are considered as EEOL. Officially you can jump between two releases, e.g. a direct upgrade from 17.3 to 18.1 is supported. However, a direct upgrade from 17.3 to 18.2 is NOT supported, as these would be three releases.
Please note that there is a difference between "officially supported" and "working". It could even happen that a direct upgrade from 17.3 to 20.1 works fine without any issues. This always depends on the used platform and the features you are using.
In general, even if you are upgrading the official way, it is highly recommended to check in your lab if the upgrade works fine, as software issues can always happen.