In the article on "Upgrading WordPress Extended" it says
If plan on upgrading across more than two major releases, please consider upgrading incrementally to avoid potential conflicts. For example, if you plan on upgrading from 2.5 to 3.0.1, upgrade to 2.7 first, followed by 2.9, and finally 3.0.1. Older versions of WordPress can be downloaded from the release archive.
I did a painstaking "one release at a time upgrade" to 3.0.1, but I'm wondering if it's really true that I can uprade from 2.1.3 to 2.3.x to 2.5.x etc...
In other words, has the upgrade.php always been coded to support at least the two previous major releases?