You don't need to start a new project. Those settings are indicative to the sequence not the project.
Now, how you do it depends on which version of FCP you are running.
If you have FCP 5 or lower:
Create a new sequence (CMD+OPT+N) choose the correct preset and hit ok. Name it as your output sequence. Double click it to load it into the timeline.
Either copy and paste original sequence clips into the new sequence, or drag the old sequence into the new sequence (nesting).
Render and you should be good to go.
If you have FCP 6:
Go into your sequence settings (CMD+0) and change the settings of your sequence to the correct settings you need for output.
Render and you're good to go.
FCP 6 will change a sequence settings even if other clips exist in the timeline, versions prior to that only changed settings when the sequence was empty.
Andy