One of the Business Intelligence systems construction metods is an incremental method, which assumes system extension by adding new sources and functionalities.
This approach enables to implement priority system elements, so users have an acces to the key functions and the most relevant data, then they can extend it by adding additional components. System extention concerns also the appearance of new data sources. It is strictly connected with ETL processes rebuilding, which allows to take advantage of many different data sources (see more: Which data sources can you connect with Business Intelligence system?).
You may exchange one source system with another without need to rebuild the whole BI solution. It is enough to correct some ETL processes (in most cases reports and managers dashboards can remain unchanged). It is a very imporatnt BI system feature which ensure the continuity of reporting in organization.