The problem is that Encore sets PAL vs NTSC when the project is created. You cannot change an Encore project from one to the other.
I hope someone who has done this has some words of wisdom.
But here is what I think is the worst case:
Make careful notes (may be screen shots of the flowchart etc would help) of the current project. Write down the time code for each chapter mark, and especially poster times if they are different. If you are not using poster frames, you may not need to worray about this: Premiere will handle with the dynamic link.
Create the new project as NTSC, add blank or template menus (they will be NTSC size and par), but I "think" you can copy paste the buttons etc. Try it with one.
Yes, all the actual linking etc must be done manually.