ahead may not be your first intranet, in which case it can be a good idea to keep the old URL active for it to take your colleagues to the right place at https://app.aheadintranet.com.
What you need to do is set up a permanent redirect from your old intranet address to the new ahead one. In terms of HTTP this is a 301 permanent redirect. This tells the involved machines that a resource has been permanently moved. After visiting the old address, the browser of the user will display the new URL of the ahead intranet.
The redirect is set up at the server that responds to requests via the old intranet address. This can usually be handled with configuration changes on the web server. As an example, on an Apache Webserver, you would have the following entry in your .htaccess file:
While for IIS, the answers on this Stackoverflow question should give you an idea how this is done.
What does not work is performing the "redirect"* on the DNS infrastructure level. You can set up a DNS record to define for a given address a place to which the traffic should be routed. However, since our servers are not set up to handle traffic from any other domain than our own, this will not work.
* Technically this is no redirect - here, you are telling the DNS infrastructure which IP address should be visited for a given domain entry.