Appendices contain information regarding the new features introduced in each version and the migration path between versions.
If you need/want to shim 3.x behavior, or partially migrate in steps, check out the Shim plugin that can help mitigate some BC breaking changes.
FC shimming can prepare your 3.x app for the next major release (4.x).
If you already want to shim 4.x behavior in 3.x, check out the Shim plugin that can help mitigate some BC breaking changes. The closer your 3.x app is to 4.x, the smaller will be the diff of changes, and the smoother will be the final upgrade.