Hi I’d like to share my experience downgrading my previously unattached, legacy category 6 travel package 7 night certificate to a new category 1-4 7 night certificate.
I called 1-801-468-4000 at 08:16 Eastern and said “Transfer reward points” at the first prompt and “something else” at the second and got through straight away.
I asked to “downgrade my legacy Category 6 7-night certificate code (870T) to the new category 1-4 7 night certificate (QP83) for 45,000 points and get a 30,000 point refund.” The agent successfully attached the QP83 to a new Category 4 booking for 7 nights. Entire process completed within 7 minutes.

Note: Agent said that QP80 would cost 75,000 points so I asked her to try QP83 which cost 45,000 points giving me a 30,000 point refund as per the announcement on the Marriott Insiders website.