RadComboBox is designed to recognize and select the items which are equal in the new ItemsSource collection. This means that if you select an item from collection1 and then you change the ItemsSource to collection2 which contains the "same" item as the selected one, the RadComboBox should automatically select this item in the collection2.
In order to tell RadComboBox which items are equal you need to override the Equals method of the data item on which the RadComboBoxItem is bound. Doing this the RadComboBox control should recognize and select the item.
If you are still experiencing this issue after overriding the Equals method, please open a support ticket and send us a sample project where we could closely investigate this behavior.