Dear Dhruva and Prakash......
Where it is mentioned that the person who have audited balance sheet and which balance sheet is adopted by Board will regine then you can not appoint another auditor in AGM.
Few Points should be kept in Mind about auditor:
1. The auditor is appointed for auditing of comany account for the Financial Year and he hold office till AGM.
2. After signing of Balance sheet for the FY for which he has given their resignation will be quite OK.
3. After the end of FY the appointed auditor will only sign documents which are related with that FY.
4. In the above case, it is a very good that such auditor has given their resignation after signing of BS.
5. If he has not signed the Balance sheet, then you will call an EGM and appoint auditor for the FY for which such old auditor has resign.
6. So, if he has signed the Balance Sheet then no need to call an EGM for appointment of an other auditor. You can wait till AGM for appointing new Auditor.
Regards