I trust this is on-topic for this forum, as I am a reseller, and am sharing an 'experience'...
Last week, my reseller account with United Hosting was migrated to a newer, faster machine on the United Hosting network (yippee!).
Matt, a representative of United Hosting, contacted me personally to get me to update my nameservers to point to the new servers (I am a reseller, though not a particularly active one).
I updated (or so I thought) the ip addresses through the control panel with my registrar. A week later, Matt contacted me to let me know that the change had not gone through! Indeed, as I discovered, the registrar did not support modifying existing nameserver ip addresses through the web interface.
Needless to say, I was concerned, as Matt had indicated the server was scheduled to be shut down! I hurriedly contacted my registrar, sending e-mail after e-mail, and calling them by phone during their business hours.
Through it all, while I as distressed at the poor quality of my registrar's support, I couldn't believe how patient and helpful United Hosting was! Not only did they keep the server running (whether the timing was just good, or they purposely left it running, I don't know), they actually e-mailed the registrar on my behalf to aid in getting the changes completed.
I may be switching registrars soon due to the headache of this (and previous problems), but you can rest assured I'm not switching hosting providers for as long as United Hosting is still around and still blowing away every other support provider (even non-hosting) I've ever been involved with.
I discovered, the registrar did not support modifying existing nameserver ip addresses through the web interface.
You should definitely find a new registrar, updating your nameserver IPs is a basic function which should be available in any reputable registrar's control panel. You could find yourself in deeper trouble if you ever need to perform other important changes to your domain account.