When you are editing full HD video and need to add a clip taken from a cell phone to make the video a bit interesting, does FCPX downgrade the settings to match the lower resolution of the cell phone footage or does it keep the HD settings (in my case 1080HD 24fps)?
If you start a new Project and the cell phone clip is the first clip in the Project, then there is a chance that the Project will take on the cell phone settings and al subsequent clips will get scaled down in the Project to fit the cell phone settings. This happens only in that particular Project, all footage in the Event and other Projects remain in their original resolution/quality. There's a preference that allows you to setup a custom preset, or for the Project to adapt the settings from the first clip that is used in it.
If you already have a 1080p23.98 Project started and you add a cell phone clip, the clip will get scaled up to match the Project and all other footage will remain untouched.