In general you want your sequence to be the framerate you want to output. Let's say your desired output is 25, then you male a sequence with that framerate. When you drag footage that's 50 in, it will look 25. Then you can change the duration to 200% and it will look smooth. The other option is to reinterpret the footage in the project window from 50 to 25, then when you drop it in your 25fps timeline, it will be half as fast and be quite smooth. You can even duplicate your footage in the project window and have have one at 50 and one at 25 (slow mo).
Hope this helps,
I have been editing on a 50fps sequence and the edit is pretty much done. If I set the sequence to 25fps now, will the edit stay intact?
Also, why set the duration to 200%? Shouldnt it be like 50% for slow mo?
You can make a new 25fps sequence and copy and paste the entire edit into the new sequence and everything will be the same. I always preserve my original sequence as a backup.
You are right about the 50%. Some apps use 50% for length some use the percentage for speed. PrP and AE use percentages opposite form each other.