Choudhury, a 1978 batch Indian Police Service (IPS) officer, will replace Ranjan as the director general of the NSG, the country's elite commando force created to tackle various facets of terrorism.
A home ministry release said that Choudhury's appointment will come into effect from the date of he takes charge.
It said that Ranjan, a 1977 batch IPS officer, will continue to hold the charge till Choudhury assumes his duties.
Ranjan's appointment as DG CISF will come into effect from the date he assumes charge.
CISF guards many of the country's premier industrial institutions and places such as airports.
Both Choudhury and Ranjan are due to retire in 2015.