Another workaround that was discussed in older threads about this subject:
http://forum.mikrotik.com/t/deprecating-old-ipv6-prefix-when-removed-or-replaced-rfc6204/66526/1
Is setting the preferred-lifetime and valid-lifetime to a shorter value, like 10 minutes. That way, if the prefix changes, the old IPv6 addresses will expire quicker. There may still be some IPv6 downtime, but it will be shorter (less than 10 minutes, or whatever).