Generally speaking, it is better to perform work based on prioritized requirements, not by technical issues. At the beginning of each Sprint (iteration) during the development phase(s) the team should work to fulfill the highest priority stakeholder requirements. This will naturally evolve the evolution of the source system interfaces, e.g. more data from system A, more from system B, and some more from system C, etc. Remember - the goal is useful, shipping software. A data model is useless unless it's shipped.