And just when you decided to ask for help, you come up with the solution, as always.
INSERT IGNORE INTO et_report_ymd
SELECT
NULL,
t.year AS year,
t.month AS month,
t.day AS day,
SUM(t.quotes) AS quotes
FROM source_table AS t
GROUP BY t.year, t.month, t.day
ON DUPLICATE KEY UPDATE
et_report_ymd.quotes = VALUES(quotes)
Note the "VALUES(quotes)" part at the end of the query instead of just "quotes".