I assume here by "fields" you mean "column names".
Keep in mind that column names in Oracle are not case sensitive unless you put them in quotes when creating the table. It's generally not a good idea to use quotes around the column names when creating the table. In other words, if you create the table like this:
CREATE TABLE FOO (
colUMN1 varchar2(10),
CoLumn2 number(38)
)
Then you can still run select statements like this:
SELECT column1, column2 FROM FOO
You can also do this:
SELECT COLUMN1, COLUMN2 FROM FOO
Also note that if you run this query, you'll see that Oracle stored the column names as uppercase in the data dictionary:
SELECT * FROM ALL_TAB_COLUMNS WHERE TABLE_NAME = 'FOO'
So there's no need to rename these columns to all uppercase. The queries you write can use all uppercase column names (assuming the tables weren't created using quotes around the column names) and they'll work fine. It's generally a bad idea to try to force them to be case sensitive.