The announcement was made 16 years to the day of the game’s launch in Japan, which was July 15th 1999. Shimomura tweeted out pictures of the album being recorded in the studio as you can see below.
Being an arrangement album, you can expect the original songs to be reconceptualized. Details on this haven’t been given yet but it could mean reharmonization, melodic paraphrasing, or orchestration.
The website for the album, which seems to be named “Promise,” reveals that it should be finished and out by September 30th this year.