You should you specific ID for the content placeholders in your .master page. The id`s must be the id`s of the placeholders you already populated with content. To get them you must go to the database of your project and find dbo.sf_object_data where the placeholder id`s are stored. there is a column place_holder and you will notice the ID`s of your placeholder (TD2D0ACAB001_Col00 and they increment in the order you placed you widgets TD2D0ACAB002_Col00 ...). If you don`t specify your content placeholders with the right id the content will not be lost, but you will not be able to edit it in the backend.
I recommit using Revision history for your pages to be able to revert to previous state if you make a mistake.
Consider making a theme if you want to just change the design of the pages as the theme requires nothing special to apply to the page.
If you have further questions don`t hesitate to ask.
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