Preserving Values

You can only reduce the last dimension.
If you use the Preserve keyword, then you cannot change the number of dimensions.
If you use the Preserve keyword, then you are only able to resize the last dimension.

Dim aCellValues_Reduced(1 To 4, 1 To 100) 

ReDim Preserve aCellValues_Reduced(1 To 4, 1 To 50)