Implement Extensibility in Account Dimension where data is already loaded.

I think you'll need Support here to have a look at your data. The fact that you have red cells at the parent level indicate that there is something funky in your structures - it could be some constraint or some other property.

Personally, I would have tackled this by creating a brand new cube, with brand new dimensions, and copying base data across with rules after a certain date - leaving the old cube untouched for historicals and moving on.