Periodically, various APIs in Ember.js may be deprecated. During a minor release, for instance when upgrading from version 1.9 to 1.10, you may see new deprecations fire in your codebase. Until a major revision such as 2.0 lands, code firing such deprecations is still supported by the Ember community. After the next major revision lands, the supporting code may be removed. This style of change management is commonly referred to as Semantic Versioning.
You can use newer versions of the CLI with older Ember apps. It's a good idea to resolve deprecations as they arise so that when a major version is available, it's easier to upgrade your app. For some significant changes, codemods may be available, so check out Release Blog Posts before you dive in.