Those who currently own the game will continue to be able to play it. It’ll also be coming to the Epic Games Store for PC. Psyonix said:

Rocket League will release on the Epic Games Store on PC the same time it goes free to play. This version of the game will be identical to the version found on other platforms, and will feature cross-platform play anywhere you play Rocket League, including between the Epic Games Store and Steam.

“Once Rocket League goes free to play, anyone who already owns Rocket League on any platform (including Steam) will be able to play and enjoy the game with full support for future updates and features. However, the Steam version will no longer be available to download for new players. To put it simply, wherever you play now (including Steam), you'll still be able to play in the future.”

The developer even has something up its sleeve so you’re not peeved you paid for the game.

“Anyone who has played Rocket League online before the launch of free to play will be rewarded with Legacy status, which includes:

  • All Rocket League-branded DLC released before free to play
  • "Est. 20XX" title that displays the first year you played Rocket League
  • 200+ Common items upgraded to "Legacy" quality
  • Golden Cosmos Boost
  • Dieci-Oro Wheels
  • Huntress Player Banner”

Anyone who has already played Rocket League online before today will also get the Faded Cosmos Boost (see above picture).

All players’ existing inventory will be carried over and the free-to-play dawn will also bring cross-platform progression to Rocket League.