[worble]

>This means if Epic ever abandons unreal, you could theoretically also just make the changes you need to support whatever your project required - as long as you didn't distribute those changes

No it doesn't, you still don't own the code so you can't just modify it and use it to develop games.

[Zambyte]

IANAL but I am fairly certain you are legally allowed to modify the source code of proprietary software and use that modified version, so long as you do not distribute it (as you couldn't distribute the unmodified version either).

The key difference between proprietary and Free Software is not actually that you can or cannot modify the source, it's that you are guaranteed access to the source in order to modify it or not. Since you usually cannot access the source of proprietary software, you usually have no legal way to modify the source for your computing.

[dcow]

It all depends on the license. There’s no umbrella ability to modify the source code of a software binary, open-source or proprietary, unless explicitly allowed.

[batty_alex]

Except you can modify it and use it to develop games, it's licensed that way

[cdash]

Completely, wrong. That is the entire reason why the source is available in Unreal. To allow you to modify it and make games.