Hi Babudass,
Migration from 2.5G to 3G is not corelated with each other because both are working on different transport tech. As 3G works on WCDMA, the access network become totally change.We can share only the antennae.Different RBS,different BSC(RNC),Separated control & payload layers are needed.Its actually migration towards all IP solution as transport. We can use media gateways for transformation of access technologies.
Much more later………….
Presently bread and butter for operator are coming from GSM. A good strategy will be use dual mode hand set (GSM and UMTS) and do load based handover between technologies.
OR
Keep your data traffic on UMTS and shift your CS traffic GSM.
Then slowly shift your traffic to UMTS and turn down GSM network.