You can do it with a macro containing the following code
Dim strDay As String, strMonth As String, strYear As String, strMonthText As String
Selection.HomeKey wdStory
With Selection.Find
Do While .Execute(FindText:="[0-9]{2}\/[0-9]{2}\/[0-9]{4}", Forward:=True, \_
MatchWildcards:=True, Wrap:=wdFindStop, MatchCase:=True) = True
strDay = Left(Selection, 2)
strMonth = Mid(Selection, 4, 2)
strYear = Right(Selection, 4)
Select Case strMonth
Case "01"
strMonthText = " January "
Case "02"
strMonthText = " February "
Case "03"
strMonthText = " March "
Case "04"
strMonthText = " April "
Case "05"
strMonthText = " May "
Case "06"
strMonthText = " June "
Case "07"
strMonthText = " July "
Case "08"
strMonthText = " August "
Case "09"
strMonthText = " September "
Case "10"
strMonthText = " October "
Case "11"
strMonthText = " November "
Case "12"
strMonthText = " December "
End Select
Selection.Text = strDay & strMonthText & strYear
Selection.Collapse wdCollapseEnd
Loop
End With