Upgrading on an M fare?

I was wondering if it was possible to upgrade with miles on an M fare.
From what I understand its not....
What is the cheapest Y fare thats upgradable?

Thanks a lot

[RichardMannion]

You can't on an M.

Has to be L, B or Y. L does not qualify for NRt route or for US citizens where the outbound sector is US originating.

Thanks,
Richard