Upcoming release of V 12

[Jeffrey Bouquet]

I'm running v12 as HEAD [ 12.0-CURRENT]

And am at a loss as to how to change it to 12.0-STABLE vs one day
running  a non-rebuilt 13.0-CURRENT  

Without risking a scenario such as:
   svn up 12.0, fail for some reason to be able to build the GPU driver, so I should
   have stuck with svn up 13.0 ??? 
As I've no second machine setup as backup.  due to moving issues. 
 
Background
At present I've various OSVERSION and UNAME_ENV set in make.conf
to work around some type of kernel mismatch issue which prevents otherwise
many ports building.

However it is working seamlessly and I wish it to continue to do so
after the day CURRENT changes from 12 to 13... 

The system was crafted from a thumbdrive ISO so still has 
/mnt/usr/freebsd-dist/base.txz ... 

Otherwise it is all good.