FWIW, if you have UA status, you can actually Same-Day Change (SDC) upon arrival at ORD and schedule your subsequent flight up to 24 hours later for free (depending on availability). The benefit is unlimited and you can keep doing so as often as you want, moving to progressively later and later flights. I've used this to have a several-day stopover in a layover city without issues (and then creatively add segments for more miles, at least back in the day when mileage was based on miles flown).

I do not believe this extends to *A status that is not with UA (e.g. Star Gold), and of course if there's a cheap ORD-YUL flight for your desired date, that may be preferable to the anxiety over having to constantly change flights and race to the airport when nothing later is available but if you do have status, it's a fun way to play the game in the right situation (and not violate the CoC, not that they'll enforce it strictly in this case).