Aoa
I think the implications of Indian military having s400 have been discussed from various angles. i am certain that Pakistan has been working on trying to find the best countermeasures that maybe employed given all the geopolitical and socioeconomic realities and constraints we face.
Having said all that, the delivery will take some time, what I am more interested in is how the US will respond to this given they very specifically mentioned the s400 system very recently.
Whether the US would make an exception for India or follow their usual route (these days) of trying to force India using threats of sanctions and such. It seems somewhat unlikely but it may even be so that the Indians have already secured the blessings for the US behind the scenes, since they do have good lobbying there.
Anyways, the reaction (or lack thereof) of the US would become known much earlier than the delivery of the system, and I am interested to see how that plays out.