Hi Dipapana,
If I understood properly, you want to move peering with AS65003 from C1(Old router) to C2(New router) and you have already create an iBGP session between C1 and C2.
--Yes i am going to move 1 peering from C1 to C2 but this will only be temporary migration till new upstream ready (3rd upstream ready for connection , Jun 2011). Then i will move back to C1. My concern is there any impact to the network (down/flap or take time to establish back the bgp session, etc) when the migration took place from C1 to C2 or C2 to C1.
--Yes ibgp session is created between C1 and C2.
It'll work but you'll have to take the following into consideration:
1. If you have only 2 BGP speakers in your network(C1 and C2) it'll work fine, but if you have more speakers (like C3, C4...), you'll have to create from each one an iBGP session to the new router, or to do so using Route-Reflector.
-- At the moment only 2 core router and run bgp. For future upgrade will do consider Route-Refltor,
2. You have to analyze the AS path received form both upstream because you can end-up using only B1 for outbound traffic because of shorter IGP cost calculated at C3 to C1 for prefixes with the same attributes from upstream. To overcome this you'll have to manipulate some of the BGP attributes, for example local-pref at C2...
--Yes this will take into consideration.