DNS Changes should be made with care, because although a change can be made quickly in the settings, the DNS change can take quite a while to filter through the internet - a process called DNS propagation.
Since the internet is made up of millions of routers and servers, the change needs to be signaled on master lists, that then cascade through ISPs and devices. DNS propagation is nothing to be concerned about, as it's an entirely normal function of the world wide web, but it can take time to complete - from between a few hours, to 24hrs or even 72+ hours or more - depending on where and how it is being updated.
The delay is due to all of the internet devices needing to report the new data. During the change, you could experience downtime, so unless you know how to do a changeover with very low TTL settings, we suggest you get in touch.
Once DNS changes have been made, you can check propagation, using a website such as https://www.whatsmydns.net/
IMPORTANT: When any DNS changes are made, it can affect delivery of your email / website / other services. If you have any doubts, please let us know prior to making any changes to you domain's DNS settings. When any changes are made to your DNS, the update must be communicated to all of the servers and routers across the globe. This process is entirely normal and is known as propagation. You should expect between 2 - 5 days for such DNS changes to propagate globally, although often is can be faster.
Important: To avoid problems, once DNS changes have been initiated, you should allow the changes to fully propagate before further changes are made. Also note that once changes have been made, you may not be able to easily revert them.
Further Reading:
What is DNS - Domain Name System Explained