Server 2008 FSMO role tranfer?

I have migrate my AD 2003 to 2008 R2, after migration it is necessary to transfer FSMO roles? if yes then m trying too, but there is some issue dont no what it is, but i can ping 2k8 from 2k3 but i cant ping 2k3 from 2k8, i have turn off windows firewall even then there is same probs can anyone tell what are the possibilities of this problem.