I am new with Premier, why cant i change frame (scene) resolutions during editing? Is there some purpose?
In Ae its easy, just from comp settings..
In Premiere, you can not change a Sequence's settings.
The alternative is create a new Sequence with the settings you prefer,
then copy>paste the contents of the old Sequence into the new one.
The answer to the question "Why" is pretty simple. We used to spend hours and hours helping people who had fouled up their sequence settings.
It is much easier to tell them to drop the video on the "New Item" button. That creates a sequence of exactly the right size.
In your case, you should drop one video on the new item button, delete it of of the timeline, then copy and paste everything on that new timeline as suggested by Joe in Post #1.
I just tried creating a sequence wth the settings of 2568X768 using 29.97 and square pixels. I started creating the sequence by using the standard DSLR 1080p30 preset, then I went into the Settings tab and changed the frame size.
I then put a large still on the timeline and exported it. Once again, I started with the YouTube 1080p settings for 29.97 and then changed the frame size. I had to unlock the link between the Height and Width to get the ratio I wanted, and to do that I had to increase the Profile to high, which messed with my frame rate, so I had to change the frame rate back to 29.97.
The export came out at exactly the right size. Note: Do not use the render files. They will just get you into trouble.