I'm in the middle of moving one of my DNS zones from another provider onto dns.he.net.
However, it looks like the old NS and SOA records have quite a long TTL (1 day), and I can't do anything to decrease the TTL at the old provider.
These records seem to have been cached by HE. So now, despite the authoritative servers for the TLD serving the new delegation, I'm still unable to get HE to serve the zone, which means my site is down.
It'd be really really nice if there was a way to ask HE to ignore its cache of such records when re-checking delegation, to avoid this scenario.
Many name server hosting providers will have a way to "purge cache" either automated or by submitting a ticket. Google for example. (https://developers.google.com/speed/public-dns/cache)
But also there's nothing that stops you from having more than one authoritative ns for a domain, so as long as you have NS delegation records for ns1, ns2, ... .he.net at the root in addition to what you have now, you should be able to add your domain as primary. There still may be some interruption if you don't replicate the RRs by hand quickly, so it would be better to add it as a slave, and when the records are automatically replicated, convert it to primary (kind of like "promoting") and remove the old NS records after everything is working, but this would require the current DNS provider to allow axfr, may be issues if your domain is DNSSEC signed, etc.