Changing member rollup without moving historical data

Hello, 

We are currently investigating the possibility of moving two UD members to a new parent in the dimension in the middle of the year without changing how historical data roll up. 

Illustrated as below, so CHILD A and CHILD B will belong to a new subtotal, CHILD 1D. 

 

Ideally, we don't want to create new members to replace CHILD 1A and CHILD 1B. 

Potentially we can create an alternate rollup, TOTAL B, to maintain the old structure, but the effort of reviewing all the reports and adding a selection parameter to switch to different rollup is a lot. 

This UD dimension leads the  planning process, which is done per PARENT A, B, C, etc. It is important to be able to see at the cut-off period data rolls up to different parents in the Forecast scenarios.   

 

Has anyone worked with this kind of change before?

Could you please share your experiences for some ideas here?

Thanks in advance!