Dns updating time
There's no guaranteed way to force the user to clear the DNS cache, and it is often done by their ISP on top of their OS.
It shouldn't take more than 24 hours for the updated DNS to propagate.
That would cause all queries to the old server to still return the proper results and after a few days you would be free to remove the reverse proxy.
To determine whether the registrar is using the name servers for your Route 53 hosted zone and, if necessary, to update the name servers for the domain, perform the following procedure: To get the name servers for your hosted zone and update the name server setting with the domain registrar Important When you change the name servers for the domain to the name servers from your Route 53 hosted zone, it can take up to two days for the change to take effect and for Route 53 to become your DNS service.
This is because DNS resolvers across the internet typically request the name servers only once every two days and cache the answer.
When you transferred DNS service to Amazon Route 53, you used the method provided by the registrar for your domain to replace the name servers for the previous DNS service with the four name servers for Route 53.
Domain registrars typically use a TTL (time to live) of 24 to 48 hours for name servers.Some users are rookies and they'll not refresh the DNS cache manually and I know we'll lose a lot of them in the first weeks after this change.