You HAVE to buy new receivers or get license from Russia to manufacture them at home.
Brahmos, MKI etc will need new GLONASS receivers. They are currently using French made GPS receivers.
Brahmos already has a GLONASS receiver.
http://indrus.in/articles/2010/08/26/brahmos_missile_04631.html
Once GLONASS is ready, we will gradually start dumping GPS within our military, including the Navy. When we get superior singnals from GLONASS, why will we want to use the civilian GPS signals again. GLONASS military signals provide 4 to 6 cm accuracy while the civilian GPS signals that we are allowed to use are accurate upto 30cm. The military GPS signals are accurate upto 2cm. So, there is no question of using GPS ever again.
Only us Civvie car wallas and town planning wallas will continue using the cheaper and better GPS. The civilian GLONASS signals aren't particularly good compared to GPS.