Suppress Auto Update of MDX Parameter Datasets in Reporting Services 2008

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 you’ve made. Bummer.
The hack to fix is to add a SuppressAutoUpdate =true to the parameter dataset. Right click the report and choose View Code. Find the dataset, and in the relevant node pertaining to your dataset, modify it. Then Save.

<Query>

<DataSourceName>DataSource1</DataSourceName>

<CommandText> …

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

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

</Query>

I found this solution here:
Suppress Auto Update of MDX Parameter Datasets in Reporting Services 2008