The thing that causes runways to get renumbered is the fact that the
magnetic variation itself, shifts. It may shift six minutes ( a tenth of a
degree ) per year. So after ten years, the local mag. variation may change
from 6° W to 7° W . So every few years, runways get assigned new numbers.
This gradual shift varies by locality on the globe. Areas that are
volcanically active can have huge shifts of 30 minutes or more per year.

"Newps" wrote in message
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You would choose the number to which the magnetic variation is taking
the runway. For example around the western US you would choose the
higher number as if you don't you'll have to renumber the runway that
much sooner.