mapping with --readonly option

Hello,

if I use the --readonly option to create a mapping, and mount it as my
root partition, can the mapping later be changed during runtime to
read-write ?

Or, in other words, if mapping was created as readonly, can I be sure
that it cannot be remapped to read-write while being mounted as my root
filesystem?

Ofcourse mapping can be closed and opened again, but I am asking here
specifically for mapping that is mounted as root filesystem, which
cannot be unmounted at runtime (as far as I know)

I am interested whether I can use readonly mapping as sort of security
feature to enforce read-only filesystem?

thank you