There is an unfortunate issue with the MDX Query Designer in SSRS 2008 Report Designer and Report Builder 2.0 where changing the main dataset overwrites the parameter datasets. This is a rather annoying issue because often you need to make manual changes to the parameter datasets. However, when you make a change to the main dataset, the MDX Query Designer wipes out the manual changes and auto-generates the parameter databases from scratch. According to the feedback I got from Microsoft, the issue will be fixed in SQL Server 2008 R2.

Meanwhile, there is a simple workaround which requires manually changing the dataset definition in RDL. Basically, you need to add a SuppressAutoUpdate designer switch to the parameter dataset, as follows:

<Query>

<DataSourceName>Viking</DataSourceName>

<CommandText> …

<rd:SuppressAutoUpdate>true</rd:SuppressAutoUpdate>

<rd:Hidden>false</rd:Hidden>

</Query>