Who knows if anyone else will EVER need to do this, but…
Recently I was editing a project for someone who had shot the footage at 29.97 fps. I started composing my edit, and then decided I wanted to use the “Modify>Interpret Footage…” option to change the frame rate to 23.976 (meaning the footage would display 80% slower).
Since I had already chopped up and composed all the footage, there was really no good way to simply “make Premiere do it” automatically, so I had to figure out a way to have all the in and out points of my 29.97 footage correspond to the exact frames of my 23.976 footage.
If one were to simply reinterpret the footage (in the Project Window) of the clips already used, the clips would not update properly on the timeline (every clip would shift inside itself and all the clip in/out points would be wrong).
I only had 15 clips (around 25 minutes of footage), so the work around I came up with was to duplicate each clip, and then place the duplicates in a bin called “23.976.” I then used the “(Right-Click) Modify>Interpret Footage…” command on each clip in this new bin to change each clip from 29.97 to 23.976 (you can do it as a bulk command by selecting all the clips in the bin and right clicking to make the change to all clips at once).
On my 29.97 clip timeline I then placed each 23.976 clip above it’s corresponding 29.97 clip. I then moused over the original 29.97 clip audio (unmodified in length) to determine the original clip length, and then used the “(Right-Click) Speed/Duration…” command to change the 23.976 clip length to be exactly the same as that of the original 29.97 clip (use actual min/sec/frames instead of percentage).
Next, one needs to make sure that the 23.976 clip on the time line matches the clip in the 23.976 bin exactly. To reiterate… when you change the speed of the clip on the timeline, it DOES NOT change the clip in the bin, thus you need to update the clip in the bin with the correct “Speed/Duration…” setting.
Before changing the length of the clip in the 23.976 bin, you will need to clear any in/out points that may have been created and left over during the original edit/composition. This means you’ll need to double-click on the clip in the 23.976 bin to open it in your Source Monitor, then right click and select “Clear In and Out.”
You could skip placing the 23.976 clip on the timeline by simply mousing over the 29.97 clip to determine the original clip length and then just changing the “Speed/Duration…” of the clip in the 23.976 bin, but I placed the 23.976 clip on the timeline so that I could verify that the 29.97 and 23.976 clips were identical frame by frame.
Mouse over the corresponding 23.976 clip on your timeline to see the full clip duration. Right click on the clip and select “Reveal in Project” to go to the corresponding clip in your Project Window (in the 23.976 bin). Right click on the clip and select “Speed/Duration…” and make sure the clip in the bin is the exact same length as the corresponding clip on the timeline (which has already had its speed altered to match the 29.97 clip).
At this point you can go to your composition timeline, double click a clip to open it in your Source monitor and then make note of the in/out points. Next double click the corresponding clip in your 23.976 bin to open that clip in the Source monitor and set your in/out points to match the 29.97 clip, and drag the clip to your composition timeline (you can grab the video between your in/out points by grabbing and dragging the little “film strip” icon right below your source window). You now have a time altered (Speed/Duration) 23.976 clip to match the regular 29.97 clip. Be sure to put your clips on alternating tracks so that when you expand your 23.976 clips back to their original speed (80% slower) there is room on the time line for this expansion.
Finally, select your clips and reset them back to 100% (from their altered times to match the 29.97 clips) and line up the in/out points of each clip.
What a pain in the ass. If someone knows of an easier way to accomplish the above, PLEASE post in the comments!