At 15% discount systems like Li Yong Ru etc, if you are just upgrading a few modules eg is it better to sell first and then buy the upgraded module(s) or just do it all in the one transaction. What I mean is, if you sell the old module you get the full price repaid and then get a 15% discount on the new one. Selling and buying in the one transaction you get the 15% discount but it would only be on the difference, no?.