I normally send the clip from the timeline with the speed already changed. When the Save As comes up I uncheck the Embed “Motion” Content.
Then render the clip set to Best and with the same codec that my FCP project is.
Then I import the new clip and replace it in the time line. Normally put it on V2 and just disable the orginal clip on V1 so it is still there if I need to make a change later.