Given that a new Statement resource will be created, we would expect that the Statement would refer back to the Account which it describes.  Thus no updates would be needed to Account.     Until Statement exists, using a DocumentReference (in R4) that has a "context.related" pointer to Account can be used to represent statements in PDF or other forms.  Those can be discovered by searching DocRef?related=Account/id.   In R5, it looks like DocRef was updated to no longer allow this, so using some extension with a custom search parameter on DocumentReference may be the best option.  We are investigating what happened to DocRef.context.related in R5, but regardless of those details, it is gone in R5, so using an extension to link from the DocRef to the Account would be the option available.