SQL & PL/SQL :: Update Statement By Joining 2 Tables

I have a table called test_renames that has two columns(new_name and old_name) and one row with the following values

NEW_NAME OLD_NAME
DRUG10 DRUG1

I have another table called SH0 that has 10,000 rows and the column of interest is called ITEM1. ITEM 1 contains the following distinct drug names:

DRUG1,DRUG2,DRUG3,DRUG4,DRUG5

Each DRUG has multiple rows associated with it.

I need to only update rows for DRUG1 and set the name from DRUG1 to DRUG10 for all rows where ITEM1 is DRUG1.

However I need to write this update statement by joining with the values in the test_renames table that contains the NEW_NAME and the OLD_NAME.

DRUG1 has 2,000 rows associated with it. The following SQL statement gives me 2,000 rows

(SELECT NVL(CHG.NEW_NAME, SH.ITEM1)
FROM test_renames CHG,sh0 sh
WHERE UPPER(SH.ITEM1) = UPPER(CHG.OLD_NAME)
AND UPPER(SH.ITEM1) IN (SELECT UPPER(OLD_NAME) from test_renames))

However when I run the following update statement, it appears that the SQL query updates all the 10,000 rows in the table instead of just touching the 2,000 rows for DRUG1. It does change all the rows for DRUG1 to DRUG10, but all the other ITEM1 values become null. What I need from the update statement is only chnage the rows for DRUG1 to DRUG10 and leave the other rows that I associated with DRUG2, DRUG3 etc. Here is my update statement.

UPDATE SH0 SH
SET ITEM1 = (SELECT NVL(CHG.NEW_NAME, SH.ITEM1)
FROM test_renames CHG
WHERE UPPER(SH.ITEM1) = UPPER(CHG.OLD_NAME)
AND UPPER(SH.ITEM1) IN (SELECT UPPER(OLD_NAME) from test_renames))

This update statement touches all 10,000 rows. I am looking to update only the relevant 2,000 rows for DRUG1.