I've never heard of an MFU being downgraded. You have a confirmed booking in CW, which is as solid as someone else buying a ticket.

Though if for some bizarre reason the premium cabins are overbooked, they're more likely to favour higher status or full-fare paying pax first.

In the climate of overbooking WT I certainly wouldn't be worried about a downgrade - you'll be more likely to get an upgrade to First!