Migrating to a new version
Godot Godot release policy a semantic versioning system, where compatibility is assumed between minor and patch releases, while major releases can break it. As such, it is generally not recommended to move projects between major versions during their development, especially if you've been working on them for a significant amount of time.
Still, new features, usability improvements, or paradigm shifts in engine's internals may incentivize you to upgrade. Below is a list of articles that should assist you when upgrading your project between versions. Each article would try its best to document every important difference and provide you with a migration path.