Just like Hyrule Warriors Legends.
Hyrule Warriors: Definitive Edition will be truly definitive when it launches on the Nintendo Switch later this year. Not only does it consolidate all the content from the 3DS and Wii U versions of the game, while also adding some of its own, it will also be bringing over the ability to switch characters mid-battle- which was previously something only available in the 3DS version of the game.
This is something that was confirmed in a Famitsu article on the game (via Siliconera). The character switching mechanic was originally introduced in Hyrule Warriors Legends, and would also go on to be included in Fire Emblem Warriors, which launched last year.
Hyrule Warriors originally launched on the Wii U in 2014, before getting a port for the 3DS in 2016. It will be available on the Nintendo Switch some time later this year, in Spring, with leaked release dates suggesting a March date.