We tested the provided project and appears that the explained behavior is caused from the SelectedValue property. When you change the ViewModel and assign value to the BeginningDate property it is not firing the PropertyChanged event because the SelectedValue of the DateTimePicker is currently the same as the BeggingDate and this is the reason the CurrentDateTimeText value is not updated.
If could you give us some more details on your scenario (What is the reason to change the ViewModel at run-time? Do you need the new ViewModel to have the same SelectedValue?), we will be able to investigate further and provide you with a solution.
I'm looking forward to your response.
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