The main differences are that the old Elle was made with a leather upper and had an LCL (leather-cork-leather) heel and sole. The new Elle is made with a micro fiber upper and a nylon synthetic heel and sole. Easier maintenance, lighter weight, visually better in person than in pictures. The old Elle also came in more width choices, A, B, C, D while the new Fusion is R(A/B) and W(C/D). The Elle is a pretty soft (not stiff) boot and, for an adult may not have enough support depending on what skills you're working on. The next model up is the Freestyle Fusion which is stiffer and might be better for an adult skater. Another choice is the Debut Fusion (boot only) model, that comes in two stiffnesses. You really should get to a fitter, even if it's a chore. It's the only way to get a good fit that would include the heat molding and blade adjustments.
As far as fit, my young daughter went from the Elle right into the Debut Fusion Standard with no fit issues at all. Boots were heat molded, but no other adjustments were necessary.