Cube History

I am trying to build a cube to monitor the sales performance. My measure is the sale amount. My dimensions include 1) Time 2) Sale hierarchy (salesman, sales office, region), and 3) Product Line. The problem I have is that when the salesman move from one office to another, the sales occured before this movement should continue to be credited to the old office. Only sales happen after the movement should be credited to the new office.
How should I implement the cube to achieve this effectively? We have more that 40,000 sales, about 500 offices, in the company to monitor. So I don't think keeping a copy of the sales hierarchy whenever there is a change is feasible.

Any inputs will be highly appreciated!

Thanks.