Does anybody know the address of the visa office in Foshan?

Hello David. Actually you need to apply for a new visa instead of coverting it. You can have this service in the local Entry and Exit Administration Department. Because I have no idea which area you are living, here are the lists of the departments in different areas:
Chancheng District: 2F, no. 215, Middle Fenjiang Road
Nanhai District: No. 1, Tianyou 3rd Road, Guicheng Street
Shunde District: Administration and Services Centre, Demin Road, Daliang Town
Sanshui District: No. 1, Xianxing Road, Southwest Street
Gaoming District: Area C, 4F, Administration and Services Centre, No. 88, Bailing Road