I have registered my 2 Nameservers with 2 dedicated IP sometime back. Now i have decided to change host.... can i re-register again and use back the same old nameserver with different IP ?

Yes, you can do that.

Contact your registrar with the new IPs for the nameservers.

Some registrars (Dotster, for example) don't even ask for NS IPs, so in that case you wouldn't have to change anything, because they look up the IP theirselves.

You will need to change the IP addresses of your nameservers with the registry. You can only do that through the registrar where your domains are managed. You will either have a control panel with which to do that, or you will need to contact the registrar and have them do it for you.

The comment on "not needing to enter IP addresses" was misdirected. You absolutely have to tell the registry what the IP addresses for your nameservers are when you create or modify nameservers, so these can be placed in the root zone files. This is at the root of using your nameservers for resolving other domains, and the root servers do not look up IP addresses themselves. If the registrar is looking this information up for you, you will want to tell them explicitly that you are changing IP addresses for your nameservers, as there is no way these guys are verifying all the nameservers for every domain under their control and submitting changes to the registry in anything resembling real time.

-t