It IS POSSIBLE to convert 4:3 projects to 16:9, but it will require some editing:
How to convert 4:3 projects to 16:9 widescreen
1. Open a NEW 16:9 Widescreen Adobe Premiere Project.
2. Rename file Sequence 01 to Sequence 02.
3. Import the OLD 4:30 Premiere Project (File>Import>filename.prproj)
4. Open the Bin Folder that is Imported.
5. Select all the contents of the Bin Folder and MOVE them beneath the Sequence 02 file.
6. Close the Bin Folder and Delete (Clear) it.
7. Double Click the Sequence 01 File and all Project Files should appear in the Timeline:Sequence 01 window.
8. Delete (Clear) the Sequence 02 file.
9. Edit motions, sizes, positions, and timings for New WIDER Screen model.
10.Save file as NEW Widescreen version.
Ridgeline Marketing Services
One of the biggest differences between PrE and PrPro is the lack of Sequences in PrE. That is why one is limited in how one handles changes, such as PAR for Widescreen and Standard Projects.
Now, PrPro CS4 has gone one further. You can have a Project (no Preset here now), and each Sequence can be different (Presets done at this level now). You can mix 16:9, 4:3, HD, SD, AVCHD, etc., but just doing a new Sequence with the proper Preset and they are all in the same Project. In CS4, if one had set up a Sequence to 4:3, only to realize that footage was really 16:9, all they need to do is set up a new correct Sequence, and just Ctrl+C/Ctrl+V that footage, all within the original Project.
One could also Nest the 4:3 Sequence into a new 16:9 Sequence and use Interpret Footage to adjust the PAR. PrE does not support Nesting, as it has no Sequences.
Steve/Ridgeline Marketing Services
Thank you for having this July 2009 thread surface again, especially since it was left with such an inappropriate conclusion.
I have given a step by step by step account of how to do this in Premiere Elements 7 and 4. IT DOES WORK, that is, converting a DV Standard 4:3 project to a DV Widescreen 16:9 one and vice versa, although the party line is that is cannot be done. All this was based on the initial thinking of Paul_LS.
See this not so long ago thread
Thank you very much to everyone who contributed to this. I have tried converting one of my 4:3 projects as described by A.T.Romano & Paul_LS in http://forums.adobe.com/message/2152227#2152227 and it works beautifully. However, I used the values 1024,720. I had a look at a 16:9 project I had created from scratch and those were the values used. I'm so pleased that I can now have projects in both aspects if desired, with very little extra effort.
Thanks again to all who contributed to the result, and to Steve from Ridgeline Marketing for reviving the original post.