Persona 5 DLC is another matter. The game only had fairly minor downloadable add-ons - costumes and the like - and these will be compatible with Persona 5 The Royal, though there's a catch. For unspecified "commercial reasons", DLC bought in Persona 5 does need to be purchased again by existing owners - but only for a fee of 10 Yen - around a cent in US currency. So it appears to be a purely clerical thing - if you drop a few cents, your Persona 5 DLC will indeed carry over. It's unclear for now if this will also be the case for The Royal's 2020 western release.