There is an application (SSRS Data Source Manager) that can fix missing data sources, and change current data sources for RDL reports and models. It can be used to change a single report / model or allow bulk changes for many reports and models.

There is also a health check feature, to detect and correct reports and models that have missing data sources. Multiple data sources for reports and models are also supported.

http://www.lecomputing.com/ssrsdatasourcemanager.html

Other uses can be- changing a selection of reports to another data source, eg moving from UAT/Development to production, or server loading issues.