ASO copy then load additive issue

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ASO cube that is copying data into a current month then loading additively on top of this data to create a total.  The problem is the load file might have a minimal amount of rows where one dim changes among the key intersections.  The copied data should reflect this change.  So the copy data might look like:  Dim1 = X, Dim 2 = L1, Dim 3 = P1

Loaded data: Dim1 = Y, Dim2 = L1, Dim3 = P1

ASO makes it a bit trickier but is there an idea someone might have where I can "stage" the copied data somewhere & reference loaded data intersections to change copied data Dim1 to equal Y?  Or is maybe SQL the best bet?  It's quick turnaround so unfortunately I didn't have a ton of time to start a lot of dev so was looking for ideas.