Fire Emblem Warriors director Hiroya Usuda has revealed that the game will give players the option to drop the overall resolution to hit a higher frame rate.
Speaking to Japan's Nintendo Dream, Usuda explained that while the game runs at 1080p, 30fps normally in docked mode, the player can choose to drop the resolution down to 720p and enjoy a frame rate boost to 60fps. This only applies to the Switch version of the game; the New 3DS version will "almost certainly" run at 30fps.
During the same interview it was also revealed that the game will allow you to switch from "Classic" to "Casual" mode during play, reviving any lost characters. However, this is a one-way deal; it will not be possible to switch from Casual to Classic.
Thanks to SLIGEACH_EIRE for the tip!