Airtel 5G is launched in India in the following 8 cities:
Bengaluru, Chennai, Delhi, Gurugram, Hyderabad, Kolkata, Mumbai, Noida.
The existing SIM card is going to support 5G. However, that may not be the case with existing handset.
Funny thing is that while 5G is launched, even 2G supporting basic voice calls is not working properly. It is difficult to talk - especially from indoors - due to frequent network issues.