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I believe that the only way to use the main balance to buy plan 79 is to wait for your current plan's validity to end.
Once the validity ends, you will be given 3 days of Outgoing 'grace validity', during which the main balance will be debited for any outgoing calls. During these 3 days, you can opt-in to have the main balance deducted for the 79 plan, by dialing USSD *567*79#.
If your validity (and grace period) has already ended, or even if you still have existing validity, you can try dialing that USSD code anyway, and see if it works.
Airtel used to allow recharging the 23 plan (later it became the 45 plan, which is now the 79 plan) via the USSD menus under *121#. It was also possible to have the Airtel Thanks app to deduct from main balance for Smart Recharges and Data Packs. But both those options (USSD and via Airtel Thanks) were withdrawn when 45 was hiked to 79.