Hi Paul,
Seems strange that a planned change would lead to a call to clear caches. Is there some client behaviour you're seeing that suggests that some caches are holding onto old data for too long? Or was there some unexpected complication in your delegation changes?
Are the zones that were being served from the old nameservers meaningfully different from the zones being served from the new ones?
It might be helpful to illustrate good and bad behaviour (in terms of DNS responses) to avoid unnecessary cache-clearing. An empty cache is a sad cache.��
Joe