This means that UA operations (UAX) intends to operate the flight, but is juggling aircraft. Given the shutdown in the northeast, there's a lot of that juggling going on. Even on normal days, aircraft substitutions are routine and you should not take confidence or be worried when switches occur.

I would strongly urge you to look at routings through IAD/EWR. If BOS remains closed, it's likely that Amtrak will be up and running and that you will have a lot more luck making it to Boston that way than sitting at the ORD Hilton.