Question: We have had issues with Mootools not being very backward compatible specifically in the area of drag and drop functionality. I was wondering if anyone has had any similar problems with jQuery not being backward compatible. We are starting to use it quite heavily and are thinking about upgrading to a newer version to start using several plugins that require it. Will we have any issues if we get rid of the older version?

Answer: 
jQuery seems to be nicely backward compatible. I have been using it for more than a couple of years now through several versions of the core and have not had issues when upgrading except a few minor ones with some plugins. I would say that the core seems to be fine but if you're using a lot of plugins you might run into some problems (but these are usually easy to fix, or the new core has that functionality built in anyway so you can just drop them).
In my experience, jQuery 1.x has been very backwards-compatible until version 1.9.

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> The jQuery 1.x line had major changes as of jQuery 1.9.0. We strongly recommend that you also use the jQuery Migrate plugin if you are upgrading from pre-1.9 versions of jQuery or need to use plugins that haven't yet been updated. Read the [jQuery 1.9 Upgrade Guide](http://jquery.com/upgrade-guide/1.9/) and the [jQuery 1.9 release blog post](http://blog.jquery.com/2013/01/15/jquery-1-9-final-jquery-2-0-beta-migrate-final-released/) for more information.
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And don't upgrade to 2.0 until you're sure that this isn't an issue for you:

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> jQuery 2.x has the same API as jQuery 1.x, but *does not support Internet Explorer 6, 7, or 8.*
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