So if I'm correctly, you've edited a video with a lower resolution than your project settings, creating black borders around your actual video. After you've exported the video you actually don't need to resize it, but need to "crop" it. AVIsynth has a crop method and you can probably Google for it and find out what the commands are, but I would take the easy route and do it at the encoding using StaxRip (using whatever codec you like). Here's a guide to it:
http://www.digital-digest.com/articles/ ... page1.html
Just keep resize at the fullest and tick the checkbox "crop borders", but you can read about that in the guide. StaxRip will do that automatically, so no worries there.
If you want to do it the hard way, you can either try to find out whether you can load Premiere Pro projects in Premiere 6.5 (where you do can change project settings) - but that's unlikely. Or create a new project in Premiere Pro with the right settings, then import the necessary files and try to copy-paste the timeline from your old project to the new one. This might not work in one go, so you might need to copy a few seconds of your timeline each time and go back and forth with it. It might be hand to mark with "*" (on your numeric pad) where you've left and use CTRL-left or CTRL-right to navigate through the markers.
Hope this helps!
PS. Yeah I had this once too, so I've learned to double check everything before starting! Some people who swear by 6.5 don't have these problems...