Hi @tommo
Firstly, thanks for your positive feedback! 
The year end lock just prevents you editing or adding to any entries in the locked period. It can be removed at any time, so if you do find invoices in an old period, you can just remove the lock, add them, and reapply it.
If you use the simplified year end tool , it will also create a year end journal which may need to be removed before you can add any additional invoices etc. However, just like you can reapply the lock, you can re-run the year end tool too.
Here’s a few other topics I thought you may find useful:
I hope that helps!