Mixed v6.5 and v6.7 PSC

Hi. We are going to start our upgrade from v6.5U3 to v6.7. We have external PSC rings, about 12 of them separated into different sites.

I know the interopability matrix says PSC v6.5 and v6.7 can run together.

I am curious though, how long I can mix the the versions in a ring topology? I certainly will not be able to upgrade them all in a week,

Thanks,,,