The 8 new subs don't just 'replace' the 2 Agosta-70s. This is a clear case of fleet expansion:
PNS Hashmat (135) and PNS Hurmat (136) date from 1978–80. They were modified in the mid 80s to fire Harpoon SSMs. They are now approaching 40 years (which is quit old) i.e. end of service life.
Much newer are the three Agosta 90B SSks. PNS Khalid (137), Saad (138) (AIP) and Hamza (139) commissioned in 1999, 2003 and 2008 respectively. These boats are armed with the Exocet SM 39 SSM and are being modified to also fire the Harpoon SSM. The latter boat came fitted with AIP, In addition, the first two of the three boats are being retro-fitted with the 200Kw MESMA AIP, which will quadruple dived endurance at 4 Kt. The first pair are now about 20 years old and should at least last another 10 years, untill 2029, 2033 and 2038, respectively.
The Pakistan government has approved the purchase of eight Submarines at an estimated cost of $5 billion. The deal is confirmed and according to sources the construction process has begun. According to MoDP Pakistan four submarines will be constructed at KSEW under ToT while the remaining four will be built in China.The sub construction work will start simultaneous at both contries and will be provided from 2022 to 2028.
The oldest Agosta-90 will be 23-29 during that period and the youngest 14-20. So, paying of the Agosta-90 will - at the earliest - start only after all new Chinese subs have been delivered. At which time new submarines would be required in order to maintain the increased number of subs for the future.
Kh-35 / 3M24
What can I say, it's a popular class of missile and hard to tell from the outside what the exact (quantitative and qualitative) performance differences are.
FL3000N
Max Range: 6,000 meter (3.24 nautical mile)
Min Range: 500 meter (0.27 nautical mile)
http://www.deagel.com/Defensive-Weapons/FL-3000N_a002627002.aspx
If PL-9C has an A2A range of 22km, it's ground launch range could be around 15km
Max Range: 22,000 meter (11.9 nautical mile)
Min Range: 500 meter (0.27 nautical mile)
http://www.armyrecognition.com/chin...data_sheet_specifications_pictures_video.html
http://www.deagel.com/Defensive-Weapons/PL-9_a002626001.aspx
https://en.wikipedia.org/wiki/PL-9
plus:
heavier warhead
all round VLS coverage