I just found an easier way to fix this...
in my case this has to deal with the following 2 lines in the XML code under the data source connection XML tag:
<DataSources>
once you remove these 2 lines
<rd:SecurityType>Integrated</rd:SecurityType>
<rd:DataSourceID>95b32f27-f5ce-4e43-87c0-58da9e27a85a</rd:DataSourceID>
And save the RDL it will open back up in design view...
Hope this helps
- Artimus