DDG 136 and 139 getting a refit. 137 and 138 will also probably going to be modernized.
Well, the PLAN has a total of four Sovremennys (as you point out). They are:
Hangzhou DDG-136
Fuzhou DDG-137
Taizhou DDG-138
Ningbo DDG-139
But they are also two different variants of the Sovremenny design from Russia.
The first two ships were Type 956A , and were purchased from Russia while they were being built and delivered in 1999 and 2000. They cost $600 million dollars (US) for both..
In 2002 China ordered two new, improved 956EM variants which were built from the start for China and they were delivered in 2005 and 2006. These improved ships cost $1.5 billion dollars (US) for both...or over twice as much.
The improved 956EM vessels had the aft AK-130 gun removed. In addition, the four AK-630 CIWS were replaced by a Kashtan CIWS short-range air defence system on each side. Each Kashtan system has a 3R86E1 command module and two 3R87E combat modules. Each combat module has two 30 mm GSh-30k six-barrel guns (range 0.5 to 4 km) and two SA-N-11 air defense missiles (range of 1.5 to 8 km) with an auto-reloader. The improved 956EM version is also the first to be armed with the newer version of the SS-N-22 missile, which was designated 3-M80MBE. There were reports that the new missile was funded by China. The newer missile increases the range from 120 km to 200 km. The air defense software was upgraded to accommodate the newer SA-N-12/SA-17 SAM system.
In 2006, extra spheres were added to the superstructures of the Chinese ships when they were upgraded with the domestic HN-900 Data link (Chinese equivalent of Link 11A/B) and SATCOM (probably the SNTI-240).
I am not sure what the new upgrades will be, but they are apparenytly being applied to both the 956A and 956EM types because we see in that photo that 137 and 138 are being worked on. DDG-137 is the Fuzhou and was the second Type 956A vessel. DDG-138 is the first 956EM vessel and is the Taizhou.
It will be interesting to see what they do to these vessels.
As I have repeatedly said, these are very capable and potent vessels, and I expect the PLAN will keep them in service into the 2030s.