Need help on a trigger or how to overlay a table field

Hi, a have to change a key-number of our db and only a few days time, the problem: From a given date on, all use of a number, say oldnr is forbidden and a new number, say newnr has to use. The oldnr is part of a table X with other (unique) fields. Many data in the running system has containing the oldnr in it and is using it in further executions. What i tried to do: Create a trigger on table X, and for _every select_ on it change oldnr via a translationtable to newnr. E.G.: select a, b, c, oldnr from X .... ---> trigger fire and so the select comes up with newnr in oldnr Who can i do that, trigger seems to work on inset/update/delete only? Thanks a lot Peter