Dear All,
Dr Solomin very kindly advised:
yum --exclude=kernel-module-openafs-2.6.9-78.0.13.ELsmp-1.4.7-68.SL4 \
--exclude=kernel-module-openafs-2.6.9-78.0.13.EL-1.4.7-68.SL4 update
works fine, and for the box where openafs was removed,
yum install kernel-module-openafs-2.6.9-89.0.3.EL
allows openafs-client to be reinstalled.
Thank you very much Dr Solomin!