If by welcome center you are referring to the checkin/owners lounge, I visited Ocean Tower at the end of December and it was open. There is also a new sales center building at Ocean Tower across from the Italian restaurant, where they are moving the sales center from Kings Land once it is complete (guessing some time this year).
Based on the exterior renovation (paint, facade around door of unit), I'd say about 1/6 of the resort has been converted to HGVC - all in the back mountain side of Ocean Tower - resort views and partial ocean views. They haven't touched the Ocean Tower restaurant, adult pool, or feature pool, which are all very old and tired. I'm hoping they refinish the feature pool to get rid of the purple willy wonka plaster water slides with something a little more upscale. Also, replacing the now defunct towel service (they have a vending machine now) with a small poolside bar would be nice. It just feels like a second class experience at the moment. The entire HWV resort feels very tired and dated. Needs a good power wash and facelift. Hilton Waikoloa Village pales in comparison to neighboring Four Seasons Hualalei, Mauna Kea , and Fairmont Orchid. It's night and day how they are kept up. At a peak in the economy you would think Hilton could afford to renovate, especially with the $250M cash infusion from selling Ocean Tower to HGVC.