Feature Request: Backwards compatibility 
Backwards compatability. I want to be able to evolve my format, change a few fields here and there and then automatically(!) get a converter that can convert files with the previous format to the new format.

Do other formats have this? Well for example I think XSD (=Xml schema) requires me to specify rules with their arcane language on exactly which fields get transformed how to where. 
This is quite manual but it would be okay for me, the big goal is to make this easy to do.

Let's say that today I want to evolve my format a little bit and move and rename a field. 
Okay I do it, then I have a new file format and new generated code. 
But I still need to support the old file format somehow... what do I do now? Either I add manual checks (=bad because now I have modifications in the auto generated code) or I keep auto generated code for both file versions around in my program (= duplicated code which I dont need). This option remains possible of course, but I think most people prefer to auto convert the old files to the new ones in their programs. It's just easier to maintain it that way, I've seen that in a commercial application. You dont want to keep all your old code around, it's a huge hassle. You constantly have to backport features somehow then. Hard to explain but those are the realities.
If kaitai can make a converter automatically, possibly offer this also as a c/whatever library then people can just auto evolve old files everytime somebody tries to use the program with an old file. Think: old project config, old project log format, old dump file etc.
It's better for the user too, he will get to work with the latest supported format without having to manually rewrite for new features.

This is actually a somewhat common problem with many commercial applications I think. Backwards compatability is a highly sought after thing but barely anybody manages to do it properly because it's so much work to do. It's not a hard thing to do, it's just a lot of manual work, every single time you want to change the format that's why auto generating converters would be great.