That seems pretty reasonable to me. And you are absolutely right that you want to retain that history, thus downgrading is the best option. Freedom or Freedom Unlimited is up to you.
On the initial query, they may deny your CSR app with that much credit line already extended. If so you can then move however much off the downgraded card, and the Marriott, to establish whatever limit you would prefer.
Not saying they would deny you for that, but just in case they do they are pretty good with approving if you're willing to move line from one card to another.
I would wait a month or so between the downgrade and the new application. There is a bit of a delay for all of their systems to catch-up and sync with the downgrade changes. I don't think it actually takes that long, it might only be a week, but better safe than sorry.