what's the original resolution and what is the new one? Show some screenshots so we can see the complexity and the video if you can.
There really isn't any good way to do this. You're going from one aspect ratio to another. If you just scale up the layers as they exist right now, all the animation will happen at the center of the screen. There will be some design considerations you'll have to make.
I decided what to do, but I'll give some explanation in case someone has better idea or in case someone has similar problem. I can't show you the project I'm talking about, cause it's on external drive I had to give to a workmate, but I'll show another one with the same parameters and problems. First I'm going to upload 4 pictures.
The first picture shows the presets at the compositions settings. The original size of the project is 720:576 (D1/DV PAL). In fact I had to do it on D1/DV PAL square pixel, cause later I had some issues when exporting, but I already had spent long hours of work on it and didn't want to go back. On the second picture you can see how my original project looks. I tried to move it to widescreen by changing the preset to D1/DV PAL widescreen - you can see the result on the third picture. It bothers me, because you can render the video with the proper resolution and size, but it's hard to work when everything looks "compressed" and you can't figure out what looks ok and what not. Then I tried to change the settings to D1/DV PAL widescreen square pixel. You can see the result on picture 4. The work area is properly stretched and the events are stretched just a little bit and the way they look doesn't bother me. This way it was very easy to figure out how much the background elements need changing. Some elements need to stay the same and some other elements need some changing (size, position, movement, effects). It can't happen with few clicks and I realize that - all elements have connection with each other according to the size of the whole video. When the size is changed, this connection looks strange and needs changing. What I mean is that it is very easy to stretch the background and leave the other elements in the center. But it doesn't look so good, so I started to move some of them on the sides, cause now I have more empty space there.
This is it.