Well there has been no official announment whether existing users of Mx4/mx3 (the ones which come with flyme) will be able to upgrade to ubuntu.
However there was a word initially that ubuntu will come as a standard update.zip which peple can use to upgrade from flyme. But wasn’t clarfied completely by Meizu. So we will have to wait for release I guess.
In my opinion, It will be a big mistake if they do not offer the ubuntu touch at least for the mx3. One of the reasons which i bought it was these rumors (not only rumors, it was shown running a test version of ubuntu). But other than that i didn’t regret it, it’s an excellent device!