Content has to be tagged with a language. Different dictionaries can be applied to content at a character level (ie different words in a paragraph can have different dictionaries assigned). So the content needs to have the preferred dictionary flagged either by the style definitions or with local overrides.
So you need to select all and change the language. You SHOULD also amend all the styles in the document so that when new content is added and styled, it doesn't change to a different dictionary. This is easiest done with a macro.
Code:
Sub SetLanguage()
'Created by Chrysalis Design Pty Ltd
'Changes all content and styles in the document to Australian spelling
Dim aStyle As Style, iLingua As Integer
iLingua = wdEnglishAUS 'or wdEnglishUS or wdEnglishUK
For Each aStyle In ActiveDocument.Styles
If aStyle.Type = wdStyleTypeParagraph Then
aStyle.LanguageId = iLingua
End If
Next aStyle
ActiveDocument.Range.LanguageId = iLingua
End Sub