External Relationships in P6v8

Attention fellow Planners, I need help to fix our level 3 construction schedule development in P6. The situation is described below:

1) There is 3 project files involved as follows:

File 1: Current EP Schedule @ Production Node

File 2: Update/Back-up EP Schedule for Month April @ Update Node

File 3: Level 3 Const Dev't Schedule @ Development Node

NOTE: File 1 has the same activities/ activity ID's with File 2 (only different data date File 1 having the latest data date)

2) At present, File 3 has external relationships from File 2 as its predecessor

3) I need to change the File 3 external relationships from File 2 to File 1.

4) Is there a faster way to change each activities external relationships/ predecessors? not going through each activity one by one?

5) There must be a way to change the Project ID of my external relationships so it would shift to the latesr EP but I have no idea how to do this.

6) Need help badly, and thanks in advance.