changing registrars has nothing to do with the DNS stuff, leave that alone for now. Just follow Doberry's instructions, keeping this in mind:
I don't know for sure, but there is a decent chance that there will be *two* emails that the old owner will have to accept. One will come from GoDaddy, and about 48 hours later the other one will come from NSI. The NSI one is very sneaky, it contains a NIC code buried very deeply near the end of the email, that the buyer will have to insert into the subject line when he sends the email to NSI.
Someone said that NSI might be requiring people to accept some long agreement before they change registrars, but I know nothing about that. It might or might not effect you.