Friends, now I want to confirm 2 different scenarios.
[1] The first one is known to all. My mom's Airtel number was recharged with Rs1498 on 26th October 2021. Yesterday I recharged her number again with 1498. Now the validity is shown as 25th October 2023 and total data is 72 GB (old 24GB + 24GB from Oct recharge + 24 GB from yesterday recharge).
[2] Three days back converted my wife's Airtel number from Corp Postpaid to prepaid. FRC done was Rs249 (or 251, not sure). Yesterday I recharged with Rs1498 knowing that there will be overlapping of plans. So Validity was till 21 - Nov -2022, with 1.5GB data till 16-Dec-2021. Today I called the customer care for the another recharge and whether it will increase validity or overlap again. CC told me to click the "Repeat recharge" button in Airtel Thanks app. So I have done another recharge today and now the validity is 21 - Nov - 2023 with 48GB data & extra 1.5GB / day data till 16-Dec-2021.
So friends who are looking for doing long term recharge (Rs1498), even though their current plan is not annual plan, just go ahead & do the annual recharges. The first 1498 recharge will overlap with current plan, but the second 1498 will increase validity (total 2 years).