If you are not changing any DNS records (A/MX/CNAME... though you can change the NS records if you move the entire zone correctly, and the old NS servers keep i

If you are not changing any DNS records (A/MX/CNAME... though you can change the NS records if you move the entire zone correctly, and the old NS servers keep it for a while too), there should never be any downtime related to a domain registrar change. DNS propagation is not completely under your control (there can be caching DNS servers that ignore the TTL of records), so you should always give it at least 24h by serving the same zone from both (if you've changed NS records). I'd instead avoid that worry by using non-registrar provided DNS servers. I would hope that TLD registries that fail to provide this are very few if not non-existent.