If your domain's nameservers are different than your client's then this will be a bit easier.

The primary thing here, is merely changing the IPs assigned to the nameservers.

For example, if your client's use your nameservers ns1.yourdomain.com and ns2.yourdomain.com then you can change the IPs on those nameservers until the cows come home.

If you're using CPanel/WHM, simply login as root or power reseller on WHM, and reassign the IPs to the nameservers you choose. You will also need to reassign the IP for each account.