If you haven't written a custom ruleset, never used a config and use Spectral via CLI only, it's very likely you are unaffected by any of the breaking changes we introduced, therefore you might upgrade your Spectral version right away. Our docs have been updated, so you can always refer to them, but to make the transition less painful, this migration guide covers the most notable changes.
Note, 3.0 version was a bit of a transition-version, without any significant changes, besides the config file we removed in 4.0. That said, the following migration is kind of 2.x/3.x to 4.x.