I don't have any source other than direct usage experience. Regarding the reasoning behind the change you can refer the urban roads section of this wiki https://wiki.openstreetmap.org/wiki/India/Tags/Highway

It says trunk highways are "These corridors usually carry the bulk of freight traffic from one end to the other, like ring roads, or roads connecting the ports."
These roads are neither ring roads or connect to ports.

It says Primary highways are "Roads that carry huge volumes of traffic between arterials, or between the city center and its highways. Can be used to differentiate between arterials that have sufficient grade separation, vs. those that do not."

The roads I changed to primary don't have any form of grade separation and mostly people walk on the road edges along with vehicles. So urban arterial road without grade separation is Primary and urban arterial road with grade separation and a ring road can be Trunk