[SMS]

How can I do the following: If the file *720.mp4 is offline, replace 720 by 480 in the URL. Thanks!

[Jiaz]

I'm sorry but that's not possible. You can try to use the eventscripter, see https://board.jdownloader.org/showthread.php?t=70525 or ask mgpai for help. He's our script master.
In case you're talking about a video site, you can also provide example links and we can check possible plugin support