The game's still new so there'll probably be an answer to this later, but I honestly can't see a use for the form changes outside of combos, and even then they're not that used. I saw a Nu player at the arcade beat some people just by staying in Dia mode. He knew the form change combos, but when a real match came he just stuck to Dia form.
Also 3C > FL > 6A link is goddamn difficult. You have to do the FL in the middle of the 3C. Too early or too late and you'll FL but the 6A won't hit them.
Is 623C supposed to be an AA? I didn't even think it had any head invul, and it most definitely not a DP to my unending sorrow. I tried using it as combo fodder in the corner but you can't do 623C > 2DD except on CH. Perhaps time will tell what its use is.
Edit: Seriously, I am digging Nu though. Dia mode is basically CT Nu with lower damage and lacking some of the bullshit properties, but man I love the old 236D. Interestingly enough the Dia mode 236DC appears from about three quarters of a full screen away, not from the other end of the screen like in CT. Perhaps that could be used as combo fodder for times when the opponent is too far away for a regular 236D to hit?
Double edit: 5C does the full sword hits, no matter how long you press it. Is this new for CP or was this in CSEX?