I will be creating cPanel accounts in WHM where I need to create subdomain address and tie it with an IP. I will then setup a new website on this address.
I would like to know, after doing this how do I then point an already existing subdomain to this new IP. Do I just go to the domain registrar and change the
A record to the new IP? Do I also need to change the nameservers at domain registrar or would changing the
A record suffice?
It depends how you want to manage your domain DNS. If you're using your domain registry DNS, you need to only change the record of your subdomain to point to your new server's IP address.
If on the other hand, you like to manage your domain DNS using your hosting provider control panel or WHM etc, you then need to change your domain nameservers at your domain registry to your preferred ones.
The easiest way is to simply change your subdomain A record to point to your server's IP address and wait for the change to propagate throughout the Internet.
I have achieved pointing a subdomain (yoursubdomain.example1.com) from a domain (example1.com) hosted in a environment running WHM (localServer) to an Amazon EC2 instance (cloudServer).
I will explain using Amazon solutions, but Route53 (Amazon Nameserver Service) steps are more or less equivalent in any other environment (create HOSTED ZONE, RECORD SETS, etc). Steps 1 thru 5 are, therefore, Amazon specific and represent background steps beyond the strict scope of this question (yet, IMO, necessary). Steps 6 & 7 answer the question regarding WHM configuration.
Step 1: Open Amazon EC2 dashboard, and create a new ELASTIC IP;
Step 2: Select new ELASTIC IP from list (18.104.22.168), click on ACTION, and pick ASSOCIATE ADDRESS from dropdown list;
Step 3: Select the instance that you want to point your subdomain to and ASSOCIATE;
Step 4: Open Amazon Route53 >> HOSTED ZONES >> CREATE NEW, insert yoursubdomain.example1.com in the DOMAIN NAME box and CREATE;
Step 5: Open Amazon Route53, select yoursubdomain.example1.com >> GO TO RECORD SETS >> CREATE RECORD SET, insert your ELASTIC IP (22.214.171.124); choose type A IPV4 ADDRESS and CREATE;
Step 6: In WHM, open EDIT DNS ZONE and choose your domain from the dropdown list (example1.com).
Step 7: Go to the bottom of the page and insert at least two of the name servers listed in your Route53 A record (in this case ns-1963.awsdns-53.co.uk. and ns-1963.awsdns-53.co.uk.). DONT FORGET THE "." at the end of the url!!!
You should be all set, use online DNS lookup tools, such as MXTools (choose DNS LOOKUP) or WhatsMyDNs, to verify if everything is set. Propagation is surprinsingly quick, because your master domain did not move, it just points new info (subdomain nameserver) dynamically now.