I am not optimistic about whatever they are replacing on the rear deck gun. I don't even think there might be replacements at all.
The second set of Project 956EM that went to China never had any rear deck guns in the first place. They also lost four AK-630s for two Kashtans CIWS forward of the helicopter deck, which is also where they are likely to put Type 1130s on the DDG 136/137 refits. There is also a bulb on the rear, can't say if its radar or satcom. I suppose the rear gun was removed in the design stage to compensate for the two Kashtan CIWS and the bulb.
In the refit, there is also an added Type 364 radar in the form of a bulb, installed on the tall rear mast, so that has its own weight budget.
When it comes time to refit DDG 138 and 139, it would be interesting if they keep or remove the two Kashtans. Replacing them with Type 1130 is more of a sidegrade, likely to yield no combat benefits, other than logistical advantages.