When can one apply for visa in person ?

I understand that to apply for visa extension in person, the earliest you can go in person is 1 month before your current visa expires.
But, when you are switching from 1 type of visa to another (for example, from work permit visa to permit free training visa), can you apply in person for the visa longer than 1 month before your current visa expires ? When calling Croydon we received different answers, 1 person said you can because you are switching from 1 visa type to another, another person said no. Which one is correct ? Thanks.