Some options:
Export a reference movie (not self contained), import that clip into a new timeline and convert from there.
Or
Duplicate your timeline and work from the duplicated timeline. Put the standards converter filter on one clip, set it up to the specs you want and select only that clip and hit copy (apple-c). Select the rest of the clips in your timeline and hit option-v (paste attributes) then select filters, then the standards conversion filter is now on all clips. Of course, this will not work if you have nay other filters on that first clip as it will paste that filter on the rest of your clips as well. the way around that is to simply remove the first filter, do the above workflow, and then place the filter back. or, you can set up the standards conversion filter,. make it a favorite, select all of the clips in the timeline and drag the filter to your timeline to place it on all clips.
Good luck and happy converting.
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