In my opinion, even potentially breaking changes can be done if a proper notice about upcoming changes is published in advance. If I was in charge, I would publish an announcement no later than in October with the following information:
- 2.pool.ntp.org has had an AAAA record since 8.6.2011
- 1.pool.ntp.org and 3.pool.ntp.org will get an AAAA record on 6.6.2022
- pool.ntp.org and 0.pool.ntp.org will get an AAAA record on 6.6.2023
This would give people sufficient time to adjust their configuration if needed.