What is the best way to export/import transaction from a Gnucash file to another Gnucash file?
The most obvious (and actually only way I can find) is to Export Transactions to CSV... and then to import from your other account. But CSV import doesn't allow to import all of Gnucash information categories, and at best can only recreate basic transaction.
For instance in the exported CSV, all the splits are included, and actually all the details of each every transactions. But when you try to import such a file, you get errors for all the lines that don't include Dates (all the splits) to start with, and so on.
You can just type the (new) account number into the 'account' field - and you are done. Yes it is that easy - provided you know the account number ... I am (in Germany) using for over ten years a worked-out SKR04 that shows the acount numbers prepended to the account's text which makes working with GnuCash a pleasure. The SKR04 from the GnuCash is practically unusable, since it does not show provide account numbers in the account's text - which makes the whole thing a blind-flight. See here for worked-out SKR04 (German)... https://www.facebook.com/GnuCash-DE-400197317114290/