The CGI was preserved well enough for the fans who got access to some of it to re-render it in HD and upload to YouTube [0]. If WB cared even a little, fully CGI-rendered scenes could have been remastered relatively easily. The only scenes that are truly un-recoverable without redoing from scratch are those composited ones.

But like the others said, the BDs are fine, by far the best the series has ever looked, even if the difference between the crisp live action and the blurry upscaled CGI is rather jarring.

[0] https://www.youtube.com/watch?v=WlDaygRhrg8