Thanks for the reply Dirk.
When the accounts in UCS were set up, it was planned to migrate to using new posix UIDs across all the linux hosts - so users were created with new UIDs in UCS. However, it subsequently turns out that there are so many files which need to be found (the long part - linux find running for hours!) and chown'ed that it would be preferable to change the uidNumber within UCS.
If UDM can't do this - would direct manipulation with ldbedit & ldapmodify be an option?