Project resize - after effects

Hi Guys, 

I'm in abit of a dialema, I've been editing a video on 4k and I've edited about half of the video so far.

The project has clips in different sizes for effect purposes, but the issue i've had with the project is that playback have been to slow. As the clips arent actually 4k quality there just about hd.

So I've reset my composition to hd 720. The issue im having now is how do I fit the clips i've already layed

in to the new composition setting without having to manually go through every clip?

is there a short cut, please help guys?

Thanks in advance kev