You can buy it again at a reduced rate.
The enhanced version of Persona 5, dubbed Persona 5 Royal, is coming later in the year (and in 2020 outside of Japan), and we’ve seen a lot of marketing around the game. It will have a pretty significant amount of new content, such as a new character in Kasumi, who recently got a big trailer. It will also, of curse, have all of the content from Persona 5‘s base game- but what it won’t have is the DLC.
While Persona 5 didn’t have any story-related DLC, it did have a large range of cosmetic DLC, some based on previous Persona and Shin Megami Tensei titles. Atlus released an FAQ about Royal, which covered several things, such as the lack of plans to port the game elsewhere. Another bummer? It will not have that DLC in one package- and moreover, if you did purchase DLC for your base game in the past, it will not automatically carry over to Royal.
You can, however, purchase it at a reduced rate. The FAQ says it’ll be “1 content for 10 yen,” with 10 yen being under a $1 USD. So it’s cheap, but regardless, you will have to buy it again (thanks to the folks at Persona Central for the translation).
Persona 5 Royal will release for PlayStation 4 on October 31st in Japan, with a 2020 release in the west.