The demo can become the beginning of your save
Nintendo’s Yoshi and the Mysterious Book demo covers the opening chapter. Its save data can transfer into the full Switch 2 game, but that promise has an important condition: Nintendo’s support instructions say to start the updated full game without existing full-game save data.
The July 8 version 1.0.2 notes document transfer support. The feature is therefore more specific than “play the demo and everything will merge later.”
Check before starting a separate full-game file
The useful order is to finish whatever demo play you want to retain, update the full game, and follow Nintendo’s import instructions before creating a separate full-game save. If you have already started the full version, do not delete progress simply to follow a general article; consult the support page for the situation that applies to you.
An import condition and a merging feature are not interchangeable. Nothing in the cited instructions establishes that two independently progressed saves can be combined. Our previous account and region warnings displaced the condition Nintendo actually documented.
A post-launch sample changes the decision
The full game’s May 21 release preceded this July demo. A prospective buyer can therefore use the sample alongside coverage of the released game, rather than evaluating only a preview build and promises about what comes later.
Transfer is valuable because it removes one kind of repetition. A player who likes the opening can continue instead of treating that time as disposable. The no-existing-save condition is the boundary on that convenience.
The chapter also provides a direct way to judge whether the discovery-focused pace suits the player. That judgment belongs to the person playing the demo; this report does not claim a hands-on evaluation or a successful transfer test.
The central advice is narrow: preserve the demo data, read the documented import conditions, and avoid assuming that starting the full game first is harmless to the intended transfer path.