Update violating constraint

Hi,
    I'm trying to do an update on a table that has a unique constraint
on the field, I need to update the table by setting field = field+1
however if this does not perform the updates on the table in a proper
order (from last to first) then the update will cause a violation of the
index *during* the update even though the table would be consistent
after the update completes. So the update fails. How do I get around
this without removing the constraint?
- Naz.