The essence of any modeling approach for product line architectures lies in its ability to express variability. Existing approaches do so by explicitly specifying variation points inside the architectural specification of the entire product line, usually with optional and alternative elements of some form. This, however, leads to a sizable mismatch between conceptual variability (i.e., the features through which architects logically view and interpret differences in product architectures) and actual variability (i.e., the modeling constructs through which the logical differences must be expressed). We contribute a new product line architecture modeling approach that unites the two. Our approach uses change sets to group related architectural differences and relationships to govern which change set combinations are valid when composed into a particular product architecture. The result lifts modeling of variability out of modeling architectural structure, consolidates related variation points, and explicitly and separately manages their compatibilities.