My thought was just to call the update code whenever the droplist was accessed which I thought could be done with the above code. Hope that answers your question about where I'm calling the code. I'm just using the combobox's function I added to its class OnSelchange.
// TODO: Add your control notification handler code here
char data  = "LVD-PTB";
CComboBox* pCB = (CComboBox*)GetDlgItem(IDC_COMBO1);
if ((strcmp(m_Dropdown, data)==0))