That is not how it will happen, if it ever fully converts at all.

Rust will first be added in a way that allows it to run on top of existing C code. That is what we are seeing here with Rust being used to write drivers.

As sub-systems get overhauled and replaced, sometimes Rust will be chosen as the language to do that. In these cases, a sub-system or module will be written in Rust and both C code and Rust code will use it (call into it).

The above is how the Linux kernel may migrate to Rust (or mostly Rust) over time.

As devs get more comfortable, there may be some areas of the kernel that mix C and Rust. This is likely to be less common and is probably the most difficult to maintain.

Nobody wants to rewrite working, solid kernel modules in Rust though. So, it seems very likely that the kernel will remain mostly C for a long, long time. There are no doubt a few areas though where Rust will really shine

No need for a fork or a rewrite.