The easiest way to change due dates into do (schedule) dates is to create a Smart List containing due tasks and projects grouped by due date, and then use bulk editing to add do dates and remove the due dates. If you're confused about the difference between do and due dates, here you can read about different dates in Marvin.
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To set up a Smart List, click on the top-right menu and select "Smart Lists". This will open the Smart List overlay.

In the overlay click on the "Add Smart List" button to open the Smart List configuration dialog. Since we need items to which we added due dates, we'll filter only tasks and projects with due dates: 

Add a title at the top and then scroll to the bottom to click Save.
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First, go to the sidebar strategy settings (accessible in strategies or by hovering over the top of the sidebar with your mouse):

 Then click to unhide Smart Lists:

Now that you can see Smart Lists in the Sidebar you can select the newly created Smart List. 

To group by dates, click on the Group By icon and choose to group by due dates:

Now that tasks are grouped we can use bulk editing to easily change due to do dates.

To select multiple items, press shift and click two tasks to select them and every task in between them. 

When the bulk editor pops up at the top, simply start typing the shortcut.

To add a do date we can either use "+" task input shortcut or schedule tasks by clicking on the "Schedule" button. To remove due date, just type "due none".  
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As you can see, upon adding do date to "today" we can see these tasks in the "My Day" view.
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