Just the rears for now......and that might explain why all the old plugs look the same........I believe the center electrode in the two plugs are different, thus the difference in appearance making it appear one is almost gone. I belive the originals were developed by NGK for this GM for this engine series. It appears to have been a dual platinum plug and the replacement rapidfire #6 are just platinum. AC Delco no longer supports the original NGK plug.
This difference should not cause any issues and you probably won't be able to tell the difference.
I only saw 2 O2 sensors, did you decide not to replace rears?