For example
A bank first introduce an internet banking with some 
options in the year 2000 and again they will add some more 
options in 2005.
in 2000 some people will use that internet banking after 
the new generation of internet banking in 2005 the bank 
will migrate all the 2000 year users into 2005.
First we have to link all the 2000 year customers in to 
20005 year application.this is migration.

Migration testing is done for a project developed for one 
platform can be deployed in another platform and testing 
the scope.