Converting a surface format

Im trying to copy a surface to the back buffer in dx9, but i get an error saying it cannot write because of the different formats, I was wondering if there was a way to convert D3DFMT_X1R5G5B5 to D3DFMT_X8R8G8B8 This seems unrealistic heh, but I cant think of any other way as the back buffer is D3DFMT_X8R8G8B8 so whatever i write to it needs to have the same format. Help? Thanks in advance Ash.