I have a column that contains a large number of unique codes. These codes contain letters as well as numbers. I am trying to append these records to a numeric field in a table which obligates me to change the letters in these codes into numbers for the field to take the data. If I don't know the exact location of the letter on each of these codes, how could I accomplish the conversion of these letters into numbers?

There are 26 letters and only 10 numeric digits. Just what rule were you planning on using to convert the letters? A better solution would be to create a lookup table consisting of an autonumber primary key and the text code value. You could then use the text value in the old table to join to the text value in the lookup table and use the numeric (autonumber) value from the lookup table to store as the numeric value in the new table.