Installing RELEASE_8 alongside RELEASE_7 on respective slices

Hi there,

(Please CC me as I am not currently subscribed with this address.)

Currently I have RELEASE_7 installed on one slice, and have copied
the file structure from there to a different slice which currently
contains the RELEASE_8 source ...

  (Actual partition names may be different as I type them from
  memory.)

  ad4s2a  /         base RELEASE_7 install /{,boot,root,usr} etc.
  ad4s3a  /current  contains copy of / & RELEASE_8 source
  ad4s4a  /misc     will contain what would have gone in
                    /{usr/ports,usr/local,home}

... swap partition is on ad4s2.

While running RELEASE_7 system, I want to install RELEASE_8 such that
it puts/overwrites the files in /current.  How do I go about that
without affecting / with RELEASE_7 installation?

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