Nintendo are one of the few companies whose games get upgraded rather than downgraded leading up to release.
Regarding a possible resolution change, we have seen this in several Switch games so far, such as Splatoon 2 and Mario Odyssey, which were lower resolution in pre-launch demos but got a boost by the time they released. For a relatively simple game like Kirby, 1080p docked and 720p portable should be achievable.