Comment by cafeinux

> I'm guessing that, if I start by importing a Japanese deck from some other source (because, for example, there's a source with high-quality notes), and then I split it into a writing subdeck, and then the original source adds new notes for new words or makes changes or whatever, that merging the results is basically unsupported.

Splitting a deck in subdecks can be done through tags, and every card has a unique ID field (usually the front, but you or the creator of a deck can define another one). Assuming tags is the only field you change, when you re-import an updated deck into your collection, Anki will match the IDs and you can define it's behavior when it comes to new or already existent cards.