Nope, you don't need an LCD and digitiser assembly unless you damage your current one. You have 3 options:
1. Replace the back only (safest procedure, absolutely 0% risk of damaging components.. This is what I'd strongly recommend you to do)
2. Replace back + front speaker grilles (again, low risk method, involves the above and prying off your current grilles and replacing them with new ones, without a full disassembly)
3. Replace complete housing. This involves by far the most risk you can ever take on your phone. You can break any of the tiny flex cables/connectors, LCD, and kill other parts of the phone.
I have tried all the steps in order, and I must say, just a word of caution, it really isn't worth the mess that can be made of it.
I was happy with the 1st method (putting a black back on my silver One, but when I saw the original black one, I really wanted my speakers to be black too.. So I did the second method, and in the process, although I was extremely careful, I poked a dent in the bottom speaker, which makes it crackle. I fixed it with tape, but it'll never sound as good as it was.. Then, there were also huge gaps on the top and bottom, between the grilles and the back, which was because I hadn't managed to get all the adhesive off. In short, to any other person, it was a perfect phone, but by now, my OCD was killing me.. So I did a full disassembly. Everything went perfect, except the screen. Now I have a One with a crackly bottom speaker and two small spots on my LCD
Now that you know what can happen (even after being extremely slow and cautious), what method do you plan to follow? Btw, are you gonna be changing colors??