Regarding protection of Localiser when conditions change from Cat I to Cat III, there should be low vis procedures in place at airports when conditions are forecast to drop below Cat I. If you have started the approach in Cat I and it deteriorates below this then it is very likely Localiser safeguards are already in place and ATC should inform you that low vis procedures are in force. If the Localiser is not safeguarded for Cat II/III then ATC should inform you.