We swapped our summer duvet for the winter one weeks ago, however the heating isn't on in our bedroom, and I do tend to keep the bedroom much cooler than the rest of the house, as I sleep better in a cold bedroom
It's still far too warm at night here for anything more than 4.5 and birthday suits. Last night was really warm so even the 4.5 got flung off for a while. If I were you I'd just keep the 4.5tog on but have some blankets available in case your visitors feel the cold.
Haven't used a duvet for 24yrs but decided to buy and try one again this year. Obviously it's not as cold over here, but gets very damp and freezing indoors in Jan/Feb, so am looking forward to it (and all the new bedding that goes with it!)
We have a 6ft 4.5 tog feather quilt but I don't know if it will be warm enough this winter now that the things are (finally) sleeping in their own beds.
4.5 tog for us and once it gets chilly we have a light fleece blanket each which we fling on and off with the change in weather. Once into winter proper I will swap the duvet for the 9 tog, but i have never needed both on despite having no heating in the bedroom. If we are chilly the fleecy blankets go on for a night or two.
I always prefer the option of layers if i'm in a guest room; very uncomfortable being overly warm.