Changing from WHV to Touriste Visa without getting out if the country

Hello !
(French here)
My WHV will end on the 24th of December, but i would like to extend it at least to the 3rd of January, or better, to pass on my 3 months tourist visa in order to finish my travel and project.
I've heard about some people who asked the immigration desk and got positive answer and didn't need to get out if the country. Do you have more detail, stories or advices ?

Thank you