Tbh, it shouldn't inherently break.
Whats going to be slightly broken is the certificate for :6443 (but you can either re-generate it, or just ignore certificate errors)
And if you only going to setup traefik for external IPv6 access, and leave the inner cluster IPv4, our services should continue to work.
If you however also want to use IPv6 internally (within the cluster), it will most likely break.