Historically, when we've introduced new badges, they've applied to works produced after the badge is released (with some complicated exceptions*). Although we haven't kept that rule front and centre, and there's technically nothing to stop you claiming a new badge for your pre-existing fanworks on the comm, we encourage you to enter into the spirit of the badges as spurs to creativity and exploration and to produce new works for new badges. :-)


* For example, "As a special introductory offer, if you post something in this round, you may choose two of your works posted to previous challenges to apply toward any of the new badges. After this round has finished, only works posted going forward will count toward those badges. Any work posted at this community in any challenge can count toward badges in the Total Posted Works category."