Wear your 400 gram boots while walking, get something like Articshield insulated boot covers for sitting still. Or a piece of wool or fleece blanket to wrap around feet. Or one of the aluminized space blankets/survival blankets to wrap around boots and legs. Make sure you can dry out boots overnight or take two pairs. Lots of extra dry socks.
I wear 200 or 400 gram insulated goretex/fabric/leather boots from 40s down to teens. If on the move most of the time I'm good with uninsulated boots with good socks down into the 20s if I can keep feet dry. Have tried 1200 gram insulated at minus 10 to 20 and they were way to warm for walking.
I don't know your level of activity, how good blood circulation is in your legs and feet, or your tolerance to "cold". I'm okay if my feet feel cold now and then, as long as they don't start getting numb.
