Achieving 256-bit encryption strength with PBKDF2 - HMAC-SHA1
As others have pointed out, you are never going to get more strength out of it than you put into it; you are just going to make it so the attacker has to go after the password instead of brute forcing the key.  And because nobody would try to brute force a 128 bit key, stretching to 256 bits isn't going to do any good. The attacker will go after the password. (I'm assuming that the salt is not secret.)

However, there is still a good reason to move from SHA1 to SHA256 for deriving a 256 bit key with PBKDF2. There is a bug in the design of PBKDF2. If you extract 256 bits from PBKDF2-HMAC-SHA1 then the attacker can go after the first 128 bits using only half of the hash compressions that you did to generate it.

This shouldn't be a problem where the attacker needs the entire 256 bits as in your usage, but there are cases where this does give a 1 bit advantage to the attacker.

So I would strongly recommend that you use HMAC-SHA256 as your PRF in PBKDF2 when deriving a 256 bit key.