A software file is not a piece of literature. When you say that an interface should not be changed, it's OK to change the comments in it, the formatting and indentation, etc. You can even change the variable names. From a programming perspective, it is still going to compile into the same .class file, so it is the same.
Personally, I think the least you should do is change the Sun comments at the beginning of each method to Javadoc, because otherwise users of the class won't know how to use it, and since this is supposed to be the basic interface to your data, that's obviously a bad thing.
Although if it is really a big worry to you, you can probably leave it exactly as it came to you in your instructions, formatting and all, and explain in your choices.txt that you did that because of the do-not-modify instructions.