Halving/doubling durations

[peter]

Hi Folks,
   Lots of older music is written in 4/2 or 3/2 time, and uses minims
   as the standard note.  I'd like to be able to enter it exactly for
   proof reading, then halve all the note values (and the time
   signature) so the main melody is in crotchets instead of minims.

   Example, the old Hymn Tune, `All Saints'

\score {
          \new Staff \relative c'' {
               \key bes \major  \time 4/2
               bes2 f g g |
               f es d4( c) bes2
           }
}

I'd like to convert this automatically to

\score {
          \new Staff \relative c'' {
               \key bes \major  \time 4/4
               bes4 f g g |
               f es d8( c) bes4
          }
}

by something like
   \halveDurations { music ... }


It'd be fairly easy to write a music function that does this,  but I
wanted to ask first if there was one already existing I could use.

Peter C