So I tried this and today was a red day but the change removed the red day now and increased my predicted FTP by 1w after dropping it yesterday after the ride by 2w.
This is the reason I am hesitant to mess about with manual FTP changes. Going back and forth can be kind of messy for the calendar in my experience (specially with no “undo” sort of functionality). One thing that would be nice is that in lieu of a predicted curve that a sandbox for playing with adjustments before applying them to the calendar would be nice.