Hi,
If the plugin was not active, then it was not able to gather the data needed to produce the reports using that data.
For such time periods, you should use the plugin you were using at the time, as its reports should still be accurate. (And then de-activate it once you’ve read its reports).
Note that the authors of WooCommerce EU VAT Assistant also offer you a migration service. So if you really want to see the data in our plugin’s reports, you can take advantage of their service, and then you will do so.
David