Modding
Since K610i and W660i have relatively similar hardware specifications and a similar mainboard, soft modifications are available to convert a K610i to a W660i by using flashing software and the firmware files of the W660i model. Such modified phone will be able to use the Walkman feature which is available to W series phones only however will be unable to use the radio function of the Walkman phone due to the lack of radio tuner in the original K610i.
This modification will revoke any remaining warranty for the phone.
Read more about this topic: Sony Ericsson K610i