We're not sure of the reason behind the decision, but Square Enix has announced that the Dragon Quest I & II HD-2D Remake version will be incompatible between Nintendo Switch 1 and Nintendo Switch 2. Both versions of the JRPG classics with modern graphics will be standalone, and save data will not be able to be transferred between console generations.
In fact, Square Enix's official website for the game reports that "no updates are available or planned. Buy your copy for the system you want to play on".
Beyond slight visual improvements, there are no other gameplay or story differences between the two versions of the game. The Nintendo Switch 1 cartridge version contains the full game, and is fully compatible if you start your adventure on Nintendo Switch 2. The Nintendo Switch 2 version is a Game Key Card.
Again, as Square Enix itself asks you to do, choose well on October 30th which version you will play.
Does this announcement from Square Enix change your choice of Dragon Quest I & II HD-2D Remake purchase?