Collocation settings tell ADSM where to put data as it is written out.
 If you want existing backup data to be affected by a change in the
collocation parameter, you would expect that data to be physically
moved from on storage location to another.  Since there was no frantic
tape mount activity following the change in your settings you can be
quite sure that your old backup data is still all jumbled together.

If you really feel that you will gain some benefit from forcing old
backup data to be collocated now, I suggest you look at the MOVE DATA
command.  I won't guarantee that this will help though since my memory
tells me that the syntax specifies a source tape volume and a target
storage pool.  Collocation also specifies a relationship between nodes
and storage pools so you will probably only achieve a bit of wear and
tear on your drives and tapes.

Murray Nicholas
IT Systems Consultant
Galeforce Information Services Pty Ltd
email:  murray AT galeforce.bu.aust DOT com