Converting All Plain Text Links to Hyperlinks

Try this:

Sub Hyperlinker()

    Dim Rng As Range

    Set Rng = ActiveDocument.Range
    With Rng.Find
        Do While .Execute(findText:="http:", Forward:=False) = True
            Rng.MoveEndUntil (" ")
            ActiveDocument.Hyperlinks.Add _
                Anchor:=Rng, _
                Address:=Rng.Text, _
                SubAddress:="", _
                ScreenTip:="", _
                TextToDisplay:=Rng.Text
            Rng.Collapse wdCollapseStart
        Loop
    End With

End Sub

After it's done replacing them all, you may have to save and reopen the Word document to have the links start functioning.