In an interview with Dengeki Online, Tetsuya Nomura, director of the Kingdom Hearts series, was able to discuss a bit about Kingdom Hearts III alongside Kingdom Hearts HD 2.8 Final Chapter Prologue. In the interview, Riku's new outfit for Kingdom Hearts III was touched upon a bit. Tetsuya Nomura confirms to Dengeki Online that Riku's new outfit has been completed, and that it's indeed based off of the Kingdom Hearts χ[chi] 3rd anniversary artwork from July of 2016. Nomura also confirms that King Mickey also receives a new outfit for Kingdom Hearts III.
The direct quote for the interview can be found below with translations thanks to Catherine Mueller (email@example.com).
--There was also a scene with Riku and the King in new clothes, and I'm looking forward to their debut in [KHIII].
The costume design is already done, and it's really nice. Just between you and me, the outfit that Riku is wearing in the [KH X] three-year anniversary illustration was used as the base.