Replace Webform Date Select Dropdowns with Textfield
Following my comment, you can edit the case start date component and change the widget to be a textfield. I was also able to disable the field on my webform.

As you have a disabled text field, not sure what you mean by changing the date format. If it needs to be done in civi, you can navigate to Administer -&gt; Customize Data and Screens -&gt; Date Formats.