We have an Odoo-instance in which we have used the Odoo accounting app for very basic purposes that the invoicing app does not provide:
- bank sync to keep track of paid bills automatically

- some minor manual tracking of debts through manual reconciliation to specific accounts

Fast-forward to now, we'd like to implement our entire accounting in Odoo. 
At the moment we have several years worth of data in accounting app, that has never really been verified. This means that our actual legal accounting data will be different, as it will have been filed properly by our accountant in the external sofware.

I have the following questions: 

Can we start a properly set up accounting in the current fiscal year (provided we adjust the entries for January and February), knowing that there is mislabeled bookkeeping data from the years before? (I am aware that reports including the previous years might provide skewed results)

Can we correct the previous years worth of accounting data somehow to get fully accurate reporting? I assume this is possible by altering the account on each journal item? 

What is the normal process of doing this if I didn't use the accounting app, but let's say I only used the invoicing app. Wouldn't I have the same issues? I would think stuff like deferred expenses registered in the external software would cause issues, because Odoo will not have labeled them as such. What is the normal flow to execute a migration from using the invoicing app to using the accounting app, including importing existing data?