Split GL from Income Statement Based on Department Code

[luckypennydax]

Hello Community. 

This is my first message on this site. I have found many answers from community boards like this and thank you all so much for your support. 
I am asking for assistance with separating some GLs (ex. on an Income Statement) based on the department code that was assigned to the GL when entering the journal entry. 
Background: I have previously mapped my Income Statement GLs and Balance Sheet GLs in excel and successfully report my reports in Power BI using measures. BUT NOW corporate is changing our GLs and there are now old GLs are being grouped together.
Example: GLs 10001 and 10003 are now 55599. I need to split the 55599 based on department code because part of that GL will be on a different heirarchy level within my Income Statement. 
Below picture is an snapshot of my Income Statement mapping. My relationship is now many to many since there are duplicates in the mapping. Not sure how to congifure my BI measures or calculated fields to split the GLs based on department code. 

Any help would be amazing.

[lbendlin]

Read about the concept of SCD - slowly changing dimensions.

Use a calendar table.