I just bought an HK vp9 long slide w/5" barrel and springs. Just slide the old one off and slide the long one on. Came with nice fiber optic sights. Use it for the range and put the original w/night sights back on for home defenseYou can now buy 5" PPQs with paddle or button releases. Much cheaper to sell your Walther and buy a 5" than to buy a kit.
FWIW, I don't know what HK is doing.