Update file definition with a LOT of records - best approach?

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Hello folks,

Our product has a main file (with a unique key on the PF, plus a LOT of
logical files) that can grow to pretty large record volume (25M to 200M
records).  We are needing to change the way we do a new release with a
database change to that file.  The question I have is this - what is the best
way to update the database file definition and translate the data to the new
definition for a large record count?

CHGPF changes the file "serially" which is terribly slow.  I have written a
process that submits a given number of "parallel" CPYF commands, but it
appears that the constraint is in building the UNIQUE access path of the PF.
Once I get to 5 or 6 jobs for a 3.5M record test data set on our
two-processor development box, I cease to get runtime improvement.  It takes
1:20 plus/minus 3 minutes.  (an hour and 20 minutes)  I have removed all
logical files so that the only index being built is the UNIQUE one on the
physical file.

I hope the question is clear.  What do you folks think?

Thanks in advance,
Michael Polutta
Atlanta, GA