I have checked under Setup-> Preferences like Accounting Preferences, Invoicing Preferences...but nothing suggests the x and y accounts are selected as defaults in Preferences

Setup > Accounting > Accounting Lists Select Cost Category from the Type dropdown

Thank you @Clay Roper - really helpful.

You're welcome! It is hard to find for sure...

Should the Account on the Item record representing the Landed Costs be updated as well to match the new Account associated with the Cost Category for Landed Cost?

If you're using items to track landed costs (as in a line item on a bill), then yes. For item receipts where you're tracking landed cost via categories, I don't think so.