For the past several years the Healthcare.gov open enrollment period has been from November 1–January 15, but a new proposed rule will update that open enrollment period to November 1–December 15.
This is the same time period that was in place for the first 9 years, so it is a reversion back to the original open enrollment period.
People will be able to update their application for subsidies through out the year, but the ability to pick/change plans will be limited to the shorter timeframe, unless you have a separate Special Enrollment Period due to life events.
If someone does not update their application during the open enrollment period they will automatically renew you to the same plan (if still available) or to the most similar plan. If your current carrier will no longer be available they will renew you to the plan most similar in price and benefits from another carrier.