Okay, I figured it out.
cur_t.execute("""
SELECT
CASE
WHEN mobilenumber ~'^0[1-9]'
THEN regexp_replace(mobilenumber, '0', '+63')
ELSE mobilenumber
END
FROM studeprofile
ORDER BY lastname
""")
In my previous SELECT statement, I picked the mobilenumber before running a CASE statement to it instead of jumping directly to CASE statement after SELECT.