The full story with the sextant replacement is in my thread:
https://www.subsim.com/radioroom/sho...=246847&page=2
In short, to answer your question, the old sextant is not suitable for astronomical navigation. Silent Hunter 5's Celestial Sphere accelerates and decelerates in various Time Compression modes, making the SH5 chronometer unusable. The new sextant compensates for the acceleration and deceleration of the celestial sphere and calculates the corrected time.