Upgrading to new releases of Mageia has never worked for me. The installer always says that upgrade won't work on my system, without providing any reason, such as not enough free space or the presence of unsupported software packages.
However, in the past I have proceeded to attempt upgrades anyway, although I don't recall if those were ever successful. If not, I would proceed to perform a fresh install. I have the iso files on my system going all the way back to Mageia 3.
I tried to perform an upgrade again with the Mageia 7 RC installer, but immediately encountered an error. The installer mounted disk partitions then displayed the message: "An error occurred. Can't call method "val" on an undefined value."
I'm still hoping to be able to perform an upgrade to Mageia 7.