According to the sourced Reddit post and further corroborated by a comment in the video itself, Aion Classic will require a subscription to access but will be free-to-play for one hour daily and will feature some 24-hour free-to-play events every five weeks. The classic server will start from update 1.2 and end with update 2.7.
Of course, all of this comes with the asterisk that there are no immediate plans for Aion Classic to make its way westward, either to the EU version or the NA version. Still, you can cross their fingers. You can also watch the Korean announcement video below.