While jQuery has seen a lot of internal changes in the two newly released beta versions, jQuery 1.11 and 2.1 there are no changes to the APIs, so developers should not face any disruption in taking advantage of  improved performance and fixed bugs.

However, as a great deal of refactoring was done for the releases of jQuery 1.9 to achieve a cleaner code base for future development work the blog post announcing the new versions points out there should be no problems with older code: "as long as you already did an upgrade to 1.9+ or are using the jQuery Migrate plugin."

As explained when jQuery 2.0 was released, the jQuery team now offer two versions of jQuery. The versions on the 2.x branch are smaller and faster but don't support older browsers - specifically they don't support Internet Explorer before IE9. Until further notice the 1.x.x branch is continuing to accommodate older browsers. Going forward the jQuery will produce dual versions each release which maintain feature parity. So 2.1 and 1.11 are an equivalent pair.

The main change in the new releases is the use of Asynchronous Module Definition (AMD) to replace the previous modular build system. In terms of performance, jQuery’s startup time has been reduced by deferring much of the feature detection code so that it runs the first time an API is called, rather than on page load. If you never call an API, you never run its code. Th team has also eliminated some situations where jQuery unnecessarily forced a page layout to occur.

The jQuery team is now using Bower for a lot of its internal dependency management, and plans to publish production versions of jQuery to Bower in the future.

If you want to know about the latest jQuery bug fixes they are detailed in the Changelog that is common to both versions.