Hi @jaylou,
Could you please share a screenshot of your flow's configuration?
Could you please show a bit more about the DateCompleted column? Is it a Date type column in your SharePoint list?
Further, could you please show more details about your SharePoint list?
Currently, there is no trigger of SharePoint connector could directly detect if the specific column of your SharePoint list has been changed within Microsoft Flow.
But you could consider take a try to create another Date type column (PreviousDate) to store the previous Date value of the DateCompleted column. I have created a SharePoint list on my side and the data structure of it as below:
Note: The DateCompleted column and the PreviousDate column are both Date type column.
When you add a new item in your SharePoint list, the DateCompleted column and the PreviousDate column are set to same date as below:
I have made a test on my side and please take a try with the following workaround:
- Add a "When an item is created or modified" trigger, specify Site Address and List Name.
- Add a Condition, left input box set to DateCompletd dynamic content of the trigger, right input box set to PreviousDate dynamic content of the trigger. Within middle drop down, choose is not equal to.
- Within "If/yes" branch of Condition, add a "Send an email" action. Then add a "Update item" action, specify Site Address and List Name. The PreviousDate column set to DateCompleted dynamic content of the trigger.
Image reference:

The flow works successfully as below:
Best regards,
Kris