Theoretically, you won't loose anything. The issue is wether or not the 5e sheet shares the same attribute names as the 3.5 sheet. My guess is that some attributes will transfer over while MOST others will be unrecognized. I would copy your characters to the character vault, create a 5e campaign and import a character from the vault. Open the character journal and have a look. I think you'll find it best to create new 5e characters manually.