Integrating Project Plans to create Master Programme Plan

Hi,

I have several project plans, created and updated by several other planners, I need to pull them together in one master schedule (a programme plan) to understand how overall programme is performing.

I tried amalgamating them all together by creating a new project- WBS for this is new project at level one is Project 1, Project 2 so on and so forth - I then copied whole set of sub WBSs in those project plans.

This master schedule once created, I included cross functional dependancies through activity relationships, this includes project activities in project 1 linked to project activities in project 2 so on and so forth.

WhenI scheduled it (f9) - all other project plans changed as a result of new relationships (which was expected anyways), because every project plan is updated on different date, so their data date is different, when I scheduled my master schedule  P6 gave me an option to choose the data date, for which I choose today's date as a data date ( as I wanted to see how it looks like today) this resulted in all other projects activity completion dates to move as well as caused negative float in massive sections of project plans.

Is there any other way create a master programme, which consist of several projects, that can be updated regularly inline with the project's current status?

Many Thanks

Ash