Can I replace my functional testing tool with jDiffChaser?

No. Their usages are different and complementary. jDiffChaser provides a way to detect the GUI rendering differences between two versions of a software. Of course, most of the time a difference can be related to a functional problem, but not all the time: for example a button can contain the same text on both versions of the software but could be rendered in another way due to system behaviour. e.g. Java can decide to display "Can..." on a "Cancel" button when the button size is too small.

When should I use jDiffChaser?

Every night, your nightly build should be followed by those comparisons to be able to quickly look at the report in the morning.

Isn't it hard to maintain scenarios?

No. One of the best practice you should adopt is to record short scenarios and to put the ones that should be played sequentially in the same directory (alphabetical order is considered when scenarios are played). It also easier to test your scenarios with short ones. And you can easily record a new version of it and replace it without having to remember a lot of actions (and we don't have to code a useless scenario actions editor that way ;) )

I forgot to define some zones to ignore in a lot of scenarios I've already recorded... Do I have to record them again?

No, you can define the zones you want to ignore during comparisons inside a external file. Just create it with the provided jDiffChaser editor, and once the file is created, put it a directory you define the path in the test configuration file (the .xml).

The zones I've added in the external file are now outdated in the new version of my "diffchased" software...

Remove the external file containing those zones as well as the link to this directory in the configuration file. The old "zones to ignore" are no longer considered during your test suite playing.

I want to test the menu content rendering on a MacOS application but the OS (look-and-)feel doesn't let the menu in the expanded state when I click on the screenshot button...

Use the timer feature. Set the screenshot timer to n seconds, click on the screenshot button, go to the menu and expand it. Wait for the screenshot to be done. And voilà.