Is it possible to alter the code of the Chili plugin, which had its latest release on July 2008, and it is licensed under the MIT license, to then license it under GPL?

As far I can see, there is no restriction about the new code being licensed under the same license. Is it really so, or is there a minimum number of changes?  

In my case, I would change the jQuery plugin in normal Javascript code that is executed in a CMS.
This essentially means that, among other things: 


The code will not use the "ChiliBook" namespace. 
The function will not be invoked as $($element).chili(), but as GlobalObject.ChiliHighlighter.process($jquery_element), where "GlobalObject" is a JavaScript object used from the CMS.
The code will allow other modules to alter the GlobalObject.ChiliHighlighter object to add functions that are optionally called from GlobalObject.ChiliHighlighter.process() when they are defined.


As alternative, as the repository I am using allows me to include code not licensed under GPL 2 or higher license when the code is not maintained anymore, could the plugin be considered not maintained anymore, as its last version was released three years ago?

It's technically legal. 

The MIT (Expat) license places a few restrictions on you. These are a subset of the GPL license. Therefore, if you relicense the code under the GPL, and keep the MIT notice, then you've satisfied the terms of the MIT license and may legally redistribute the code.

Note that you may not claim copyright ownership; you'll have to acknowledge the original copyright. 

[edit]
Some people don't seem to understand how F/OSS works in conjunction with copyright and license law. Everything starts with copyright, if only because that's the default. Under the copyright doctrine, the author gets the right to make copies of source code. Under the MIT license, that right is granted to me, as well as the right to recursively grant it to others. Note that the MIT license explicitly includes the right to sublicense. Quoting: "the rights to use, copy, modify, merge, publish,distribute, sublicense, and/or sell"

When I sublicense code, I cannot grant rights that I didn't originally have. In the case of the GPL, I am explicitly forbidden to sublicense only some rights. But neither in law nor in the MIT license do I have an obligation to sublicense all rights as a whole.

Therefore, the MIT license grants me the explicit right to sublicense rights, and neither the law nor the MIT license prohibits me to sublicense only some rights. Also, neither restricts the form in which I do. Therefore, I have the undeniable right to grant a GPL sublicense on that code.
