DNS and Nameserver changes can take up to 24 hours to propagate but generally do so much quicker. It is the nature of internet traffic which causes DNS propagation to happen gradually from place to place. I.e- Some users may see your website responding from the old IP while others users in different geographic locations may see your website responding from the new server.
The time taken for all internet service providers across the world to update their DNS cache records (temporary saves) to know & detect that the IP has changed for you domain is known as DNS propagation time. You can track the propagation of your changes from https://www.whatsmydns.net/
If you require further information/assistance on this, please submit a ticket and a member of the team will be happy to assist further.