Hello
MRP areas are only relevant for planning, that means, they will not affect existing stocks or costing.
You will have to extend each material to the desired MRP areas but you can use report RMMDDIBE for mass processing.
Existing documents which are assigned to an storage location will be automatically assigned to the respective storage location MRP area.
When activating MRP areas you should also run the planning file entry conversion on transaction OM0F.
In addition, you should create a scope of planning including your plants and the MRP areas on the desired planning sequence and also change your MRP variants to plan using the scope of planning.
BR
Caetano