In contrast to other industries, the Snell Memorial Foundation keeps a tight hold on the release schedule of helmets bearing their certification. Traditionally, October 1st of the changeover year is the release date manufacturers are held to. In other words, October 1, 2015 for the SA2015 helmets, and October 1, 2020 for the SA2020 helmets.
Snell released the SA2020 homologation materials to manufacturers in April 2020, signaling that manufacturing of the new helmets could begin. Sales are still disallowed until October 1, 2020.
October 1, 2020 is the official release date for SA2020 helmets. No SA2020 helmets may be sold or delivered from any manufacturer before that date. If you need a new helmet before this date, you will need to purchase an SA2015 helmet, with many at significant discounts now.
Disclaimer: Just because a helmet hasn’t expired per your sanctioning body doesn’t mean it shouldn’t be considered for replacement. Sunlight exposure, sweat, humidity, and multiple other factors degrade your helmet’s ability to protect you over time. The rate of deterioration will vary from case to case, but a good rule of thumb is every 5-6 years. If you’re using it twice a year, expect it to degrade a little more slowly. Conversely, if you’re using it 2-3 times a month, expect to replace your helmet more often.
This answer will vary depending on your sanctioning body, but our experiences at DiscoveryParts are generally that the current and preceding standard are accepted by most sanctioning bodies. In other words, we expect most sanctioning bodies in the US to accept SA2015 helmets through 2025, BUT ALWAYS CHECK THE RULEBOOK FOR THE ORGANIZATIONS YOU INTEND TO DRIVE WITH.
In short, not much. The SA2020 standard is a revision of the SA2015 standard to bring it more in line with the FIA8859-2015 standard. Not all helmets will carry both ratings, so check with your sanctioning body regarding which rating you need (US-based events generally require Snell certification).