You had earlier said that it was a mechanical construct within the engine, and that the Russians did it so we don't cross MTCR limits by ourselves, but we are doing this range extension by ourselves, the Russians are not involved.
Brahmos Aerospace used the guidance software to shut down a fuel tank. So the engine was starved of fuel much earlier than the Oniks. And as soon as the fuel is starved, the missile self-destructs. Now, they've removed that restriction. There is no mechanical construct in the engine that blocks range. I have no idea who told you this. The current modification only requires a bit of re-engineering in the fuel lines.
You see, if it was modifications to the engine, there was no way we would be ready for a test within 8 months of joining the MTCR. We would have needed a brand new engine from Russia first, and that would have taken multiple years, involving the release of development funds followed by prototyping, something that we will have to follow for the 800Km extension. And obviously, there would have been no way to enhance the range in the field for operational missiles either, making the exercise pointless.
Anyway, the good news is we can upgrade the airborne version to 800Km much sooner than possible, while the standard Brahmos-A can come with the 600Km extension right away.