While the Shanghai-Beijing HSR was designed for 350 km/hr operation, it has in fact been operating at 300 km/hr for several years. You'll recall speed was reduced on all HSR lines due to concerns about construction issues.
In any event, they've just increased the speed on some departures on that line back to 350 km/hr. These departures carry a surcharge.
But no matter, 300 km/hr or 350 km/hr, they're all HSR.
Which trains were speed up? I just checked Shanghai-Beijing on 12306, and the fastest trains are still 4:48 (G2, G4, G8). Incidentally, G8 is a welcome addition to the schedule because it makes it possible to work an entire day before leaving.
Regarding the decrease from 350 to 300, this happened after that accident in ~2010, and safety was the stated reason for the change. However, since that accident had nothing to do with the speed of travel, I have long suspected that saving money was the --behind the scenes-- reason. During testing of the Jing-Hu Line, I recall reading that some trains were run at very high speeds (e.g. 450).