Possibly, they are trying to fit some new equipment to the last two Type 055C, which are directly outsourced from the Type 055D.
Like, they might have changed the problematic 100mm main gun of Type 055C in favour of a new gun.
No, I doubt that is the reason for the delay. There is nothing to suggest they are replacing any components from 052D, and they certainly wouldn't be replacing the gun, which despite the better performance of the 130mm gun, is not enough to warrant the complicated procedure of replacing the 100mm gun already on board.
More likely, the last two 052Cs are simply being sea trialled very thoroughly so when they are officially "commissioned," they will end up being far more battle ready.
On the flip side, the reason 172 was commissioned so early was probably a mix of political reasons, as well as to give the navy greater involvement in the first sea trials of the class (which is similar to what happened with the first two 052Cs in the mid 2000s).