I have been informed by a co-worker that this story is not entirely true...

While the actor is no longer doing the part of R2, it will not be CGI. Here's the story that started it all.

Evidently someone saw this and extrapolated that the droid would be inserted digitally. But they are absolutely, definitely 100% positively using an R/C version for the film: There's also another shot of a crewman with a radio, controlling R2, but I can't find the picture right now.