Revisiting the kernel's preemption models (part 1)

There is no such thing as a "system RT mode".

The kernel just provides mechanisms for tasks to be scheduled with various RT properties (e.g. RT-Fifo, RT-RR, deadline sched, locking latency guarantees, etc..).
But that does *not* mean that every task suddenly becomes a RT task. In fact, in a RT system almost all tasks usually still run in normal scheduling mode.
Normal tasks on a RT kernel basically just behave like tasks on a full-preempt kernel. Just with a little bit more overhead here and there. (I'm simplifying for the sake of briefness)

A task has to opt-in to being RT. (And selecting a RT sched class is only part of that. There are many many many more things to consider.)

And that's why it doesn't make sense at all to switch between full-preempt and RT-preempt during runtime. That would just add even more overhead with no added benefit. If your task doesn't need RT, then don't schedule it in a RT class.