The temporary concierge building will be in Tahiti. It is the lagoon view facing longhouse closest to the TTC. It is my understanding there will be a check-in desk area in the common area on one of the floors and the food service will be either on 2nd or 3rd floor. I'm not sure how they are going to accomplish the food service, but it will be interesting to see.
Tahiti is a larger building than Hawaii and the lagoon view rooms are equal too, some even say better, than those in Hawaii. Obviously, there won't be a "castle" view lounge--unless the food service dining tables are actually placed in a lagoon view room--who knows, I suppose that could be done BUT I have no basis for that idea--it's my own. Tahiti rooms also have the double sink vanity and larger bathroom area. The rooms, if I'm not mistaken, are actually a bit larger as well.
I've stayed in Tahiti before (then known as Moorea) and had a lagoon view--2nd floor I believe. The castle was clearly visible. The only drawback in my experience was the water parade wasn't as visible from that particular room--we could see it, but not like you can from Hawaii.
As far as the service is concerned, I am confident that aspect will remain unchanged--if you loved it (or liked it even) during past stays, I doubt you will be disappointed. It may be a bit more inconvenient to the GCH and pools, but only by a couple of minutes. I guess as to whether or not it will remain "worth it" to you--only you can decide. We're remaining faithful and will make our return in June--hopefully any bugs will be ironed out before our arrival.
Tikiman's site has the news about the refurb--I got my information from there and the concierge staff. Hope it helps in your decision making process.