This article is from our resident Drupal 8 core maintainer Nathaniel Catchpole (@catch) who is helping to build Drupal 9.

Drupal 9 will be the first major core release to have a continuous upgrade path meaning that Drupal 8 contributed modules and themes, and actual Drupal 8 sites, should be able to upgrade smoothly from Drupal 8 to Drupal 9, with only slightly more disruption than a Drupal 8 minor release. Since this is the first time that Drupal has ever attempted to provide a smooth upgrade path between major releases, we understand it can be hard to imagine what it will be like. 

The best way to feel confident will be to actually understand the changes that will be landing in Drupal 9 over the next few months, and when you should try to test it, use it to build new sites and to update existing sites.