"Although" implies a contrast to a statement that is true but incomplete or questionable in some sense. E.g., "Many customers upgraded their accounts, although some did it illegally." The word you need to complete your sentence is a simple contrast. E.g., "The new service was expected to be a success; however, very few customers upgraded their accounts." "However" or "but" are the conjunctions we might expect here. If this answers your question, please vote me up.