It seems it’s impossible to do it from the user interface. But I found here that it could be done using a VB Macro:

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Sub ChangeAllTablesToNormal()
Dim myTable As Table
   For Each myTable In ActiveDocument.Tables
      myTable.Select
      Selection.Style = ActiveDocument.Styles("Normal")
   Next myTable
   ActiveDocument.Repaginate       
End Sub

You may try to change ActiveDocument.Styles("Normal") to ActiveDocument.Styles("Tables Normal") as suggested by Abhishek if it doesn’t work for you.