For instance, at the time of writing this, Germany's VAT rate is due to increase from 16% to 19% at midnight on 31 December. What are our options?

This is the simplest solution but it does mean you doing some work at a time when you'd probably rather not. There is also the small chance that you'll do it a few hours early or late and someone buys a ticket at the wrong tax rate.

Just go in to Settings, choose Tax types and edit the VAT rate to 19%. None of your previous sales will be affected. It will only change the price of tickets sold from then on.

Good: least amount of work

Bad: you have to work at midnight on New Year

Option 2) Prepare ahead of time

Add a new 19% tax rate in addition to your existing 16% tax rate.

Then duplicate all of your tickets:

Change your new tickets to use the new tax type.

Then schedule your existing tickets to be available until 11:59pm on 31 Dec and your new ones to become available at 00:01am on 1 Jan.

Good: stress free New Year

Bad: more things to change and you end up with two of each of your ticket types. The attendees won't notice that but it will affect your reports.