Since I have no knowledge of your locale this may be of limited or maybe no use, so feel free to ignore if it isn't helpful.

kind of depends. Is that a rare overnight low or will you actually need to be riding in those temperatures? If it is a once or twice in the trip overnight low and maybe a morning or two of cold riding I'd get by with my summer gear and wear everything I had along if necessary or even take a morning off here or there. If that is a normal daytime high temperature that is another matter entirely.

I figure I can get by with my warm weather gear when the daytime highs are generally 50F (10C) or above even when the overnight lows get into the teens once in a while and hit freezing regularly. When it is regularly freezing all day and 0F (-15C) at night that definitely requires entirely different gear for me. I'd either have to carry a little heavier gear the whole trip or manage to supplement or swap somewhere along the way. I'd think clothing might be easy enough to just buy some more no matter what country you are in. Your sleeping bag may be harder. Taking a warmer bag from the start, managing to get a swap for a warmer one, or getting an over bag somehow would seem to be the options.