With the MDX Query Designer in SSRS 2008 Report Designer and Report Builder 2.0, changing the main dataset overwrites the parameter datasets, which will kill any manual adjustments made. Bummer.
The hack to fix is to add a SuppressAutoUpdate =true to the parameter dataset:
<Query>
<DataSourceName>DataSource1</DataSourceName>
<CommandText> …
<rd:SuppressAutoUpdate>true</rd:SuppressAutoUpdate>
<rd:Hidden>false</rd:Hidden>
</Query>
