My first goal is receive security updates from 6.7 upstream.
If we release 6.7 Beta immediately everyone can upgrade to it and receive security updates. We just need some tests to be sure that upgrading the base system from 6.6 to 6.7 works.
There should not be changes to services behavior during 6.7 Final development, thus I assume an upgraded production system will be stable enough if we’ll follow the usual “rolling release” process.
I expect biggest changes are applied to the ISO installer and affect only new installations.