Hello again Rob,
Phew, not sure we ever faced a scenario like that. There are 2 possibilities that I can quickly think of.
After you changed the event details you will need to go to your WooCommerce orders and go to every order one-by-one and resend the tickets. You can find this in the Order Actions box, looks like this. This is a bit cumbersome, especially with so many tickets and orders.
The other less cumbersome option would be with the use of this extension, which adds the event (and attendee) details in the WooCommerce emails. Once the extension is in place and set up go to your WooCommerce orders, select the affected orders and bulk change their status to something else then completed. Then again select them and bulk change to completed. This will sent out the completed order emails to the users, which should now contain the event information.
An extra note, if you change the event details the QR codes will not be changed. The only change if you create a new ticket or a new event or a new purchase.
I hope this will help. And let me know if you have any more questions.
Cheers,
Andras