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When changing the site code for SCCM clients, it is normal for the collection to go on evaluation since the site code is getting changed. The evaluation process evaluates the membership rules of a collection to update its members, and the time it takes to complete depends on the number of devices being migrated.
To monitor the evaluation process, you can use the Collection Evaluation Viewer (CEViewer) or view collection evaluation in the Configuration Manager console.
References:
- Collection evaluation in Configuration Manager
- Best practices for collections in Configuration Manager
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