Michael is a faster typer - but anyway:
Create a new (empty) sequence with the correct settings.
Return to your original sequence, and click anywhere on the time line and type Command + A to select all the clips on you timeline. Once selected, type Command + C to copy them.
Now go to your new (empty) sequence, and click in the timeline and type Command + V to place all your clips. If the aspect ratio or frame size is not correct, click anywhere in your newly pasted time line and type Command + A to select all the clips. Once selected, Control-click on any one of the clips and from the drop-down menu, choose Remove Attributes. When the Remove Attributes pane open, check the box for Basic Motion and Distort (if available to be checked) and click OK. All your clips on the timeline should now be resized.
Does this help?
Message was edited by: Meg The Dog
This will only work if your not changing the frame rate.
Create a new sequence with the correct settings. Edit one representative shot into the new sequence. Does it look and play right? If so, select the clip in the timeline and hit copy (command-c). Then delete it from the sequence. Make your old sequence active and select all (command-a) and then copy (command-c). Make the new sequence active and hit command-v. Your edits should appear in the new sequence although the image won't look correct. Now, select all (command-a) and and hit option-v (paste attributes) and just check motion. That should adjust pixel dimensions properly.
Post back if this doesn't work, or if it ins't clear.