How can one reliably update a LOB column with data from another LOB column in Oracle?

Question

I have tried: UPDATE "SCHEMA".TABLE SET LOB_COLUMN = (SELECT LOB_COLUMN FROM "SCHEMA".TABLE)

I have tried:

CREATE OR REPLACE PROCEDURE COPY_LOB_DATA IS
    OLD_BLOB BLOB;
    NEW_BLOB BLOB;
    BLOB_LENGTH NUMBER;
BEGIN
    SELECT LOB_COLUMN INTO OLD_BLOB FROM SCHEMA.TABLE WHERE OLD;
    SELECT LOB_COLUMN INTO NEW_BLOB FROM SCHEMA.TABLE WHERE NEW;
    SELECT dbms_lob.length(NEW_BLOB) INTO BLOB_LENGTH FROM DUAL;
    dbms_lob.TRIM(OLD_BLOB,1);
    dbms_lob.copy(OLD_BLOB,NEW_BLOB,BLOB_LENGTH);
    if (dbms_lob.compare(OLD_BLOB,NEW_BLOB) = 0) THEN
        commit;
    ELSE 
        rollback;
    END IF;
END;

and I've tried using cursors. Each method seems to work four times out of five, but every so often we'll encounter a field that just won't budge. the largest of these would be around fifty megs, but usually it's one of the smaller ones.