switch between silver , gold sellers

[sportphoto]

Hi
i have 3 types of sellers = Bronze,silver,gold
register sellers automatically will be Bronze.
now i can not find out how my seller's can request to change to silver or gold,
also i like to know can i charge them for this change ?

[admin]

I have no anwer...Only a site admin can change the category. There is no option for charging.

[sportphoto]

so it will be good if sellers can send request for admin to change they account. because no point to have it