Hmm. It depends on what you call happy. If it's in terms of life and death, I'm fine if one of LSS or HG (or both) dies in the prequel. I just don't want one of the characters to love the other (but the other doesn't reciprocate his/her feelings), which causes the heartbroken original character to switch to another random guy/girl and then slowly falls in love with that guy/girl, which means that they won't end up as the OTP (one true pair) for their section of the story. (Mel, I hope I'm not confusing you!)
E.g. 1 "Liu Shi Shi + Hu Ge = Liu Shi Shi confesses, Hu Ge rejects for some reason. ---> Liu Shi Shi heartbroken, along comes another guy to heal her heart. Liu Shi Shi + random guy = falls in love with each other. Hu Ge realises he still loves Liu Shi Shi, tries to chase her back, but now she rejects him. ---> HG left all alone, while LSS and random guy are happy forever."
^^The above example (taken from Da Mo Yao) is a definite no no! Must not happen in this prequel between Miu Jing Hung and LSS.
This is quite a long post, isn't it?