I have noticed that sometimes people have to use multiple versions of jQuery in the same page (See question 1 and question 2). I assume people have to carry old versions of jQuery because some pieces of their code is based on an older version of jQuery. Obviously, this approach causes inefficiency. The ideal solution is to refactor the old code to use the newer jQuery API. I wonder if there are tools that automate the process of upgrading a piece of code to use a newer version of jQuery. I've never written programs in in either Javascript or jQuery. So, I'd like to hear from programmers experienced in these language about their opinion on this issue. In particular, I'd like to know the following.

1. How much of problem it is to have to load multiple versions of jQuery?
2. Have you ever had to load multiple versions of any other library in the same page?
3. Do you know of any refactoring tools that helps you migrate your code to use the updated API?
4. Do you think such a refactoring tool is useful? Are you willing to use it?