In order to maintain dignity of particular variants and ensure optimum occupancy of a particular category of train, there should be some speed criteria, without satisfying which that particular category of train can't be run (exception to geographic/administrative/altitude/soil/terrain constraints be allowed)
Vande Bharat : 80km/h avg speed and at least 1 tier1- tier2 city en route
Shatabdi/Tejas ...
more... : 65+ avg speed.
Jan shatabdi : 55+ avg speed.
Rajdhani : 60+ avg speed/ capital to capital
Duronto/Sampark Kranti : 55+ avg speed
The likes of bsp ngp VB be converted to jan shatabdi, dli kgm/rmr sk and shatabdi converted to Intercity etc.