Cascading updates to FKs with unique constraints

Greets,

I have a table that holds unique accounts referenced by many tables and
occasionally we need to merge duplicate accounts together. In the end,
we want all related records from both accounts to be merged down to a
single account ID.

What I would like to do is use ON UPDATE to cascade the change in the
account table ID to all other tables. However, there is a unique index
on account IDs preventing me from something like:

UPDATE accounts SET ID = IdOfNewAccount WHERE ID = IdOfOldAccount;

where this query would generate a duplicate row in accounts. Are there
any other options besides writing the queries by hand (there are a
decent number of dependent tables) to update the dependent IDs with
IdOfNewAccount?

I think the answer is no, but thought one of the gurus on the list might
have a more creative answer. Thanks!

Brian