Unity 7 is a direct continuation of the Unity 6 line. Upgrading from Unity 6 to Unity 7 is designed to feel like the 6.x upgrades you've already done.
As with every 6.x release, there will be some breaking changes: obsolete APIs becoming errors, packages moving into the engine, platform minimums moving forward, and deprecated features being removed. What we don't expect is anything that forces you to rebuild your project, rewrite your codebase, etc.
You'll get the same advance notice you've had throughout 6.x. We publish a thread on Unity Discussions during alpha covering planned breaking changes and keep it updated through the release, so you can plan the work — and tell us if we've missed something about how a change affects real projects.