dipen have a look at yahoopops and do tell me if it works for you. If I my computer starts working this week I will check it and tell you the results.
Download and configure yahoo pops and download all th e mails from your yahoo acount and using email client(at least thunderbird, that what I tried on) you can select all mails and forward them to your gmail account.
And for receiving the mails on yahoo is considered do the following:-
Go to settings in gmail ->Accounts ->Send mail as -> When I receive a message sent to one of my addresses: -> select Always reply from my default address.
Hope this helps you.