Filipado,
what about something like

UPDATE A
SET GroupCode = :NewValue
WHERE A.Code = :Code AND
NOT EXISTS (SELECT 1 FROM B WHERE B.SomeField = A.Code) AND
NOT EXISTS (SELECT 1 FROM C WHERE C.SomeField = A.Code) AND
NOT EXISTS (SELECT 1 FROM D WHERE D.SomeField = A.Code)

But be a bit careful with your transactions, so that no-one can insert the
old value into B, C or D simultaneously with someone else updating A.

HTH,
Set

BTW, this question would probably have gotten more replies at
ib-support@yahoogroups.com