what is the differnce between updgrade and migrate related to Websphere?

Migrate means changing to next release:
eg: 5.1.0.0 to 6.0.0.0

this is migration..

upgradation means applying fix or refresh packs...
eg: 5.1.0.0 to 5.1.2.0 (refresh pack)
5.1.2.0 to 5.1.2.31 (fix pack)

applying fix packs and refresh packs is upgradation