Here is the reply I got from frro, but some parts are still not clear. I didn't specifically ask for Canadians.
Sir/Madam,
01. As per Ministry of External Affairs (MEA) guidelines a grace period of three months (90 days) is allowed from the date of acquisition of foreign citizenship, for the purpose of travel on Indian passport. For further clarification and fresh guidelines if any, in this regard you are advised to approach Indian Embassy abroad/concerned Regional Passport Office (RPO) in India.
Regarding surrendering of passport, foreign national is advised to approach Indian Embassy abroad/concerned Regional Passport Office (RPO) in India.
02. Further, as far as departure from India is concerned, any Indian acquires nationality/passport of another country then he/she has to obtain "Exit Permit / Visa" from the FRRO/FRO through online application at
https://indianfrro.gov.in. Further, if "Exit Permit" is issued, the foreign national must depart within the validity of the Exit Permit.
Note: Grant/denial of Exit Permit/ Visa related services is subject to submission of surrender certificate of Indian Passport, Indian voter identity card (if issued by the ECI), etc. as well as usual checks and verification.
03. A foreign national is not allowed to stay without a valid visa in the country and indulge in activity other than for which visa has been issued. Violators of visa rules will be prosecuted according to the Law of land. Hence, every foreign national is advised abide by the Law of the land and instructions / guidelines endorsed on the Indian Visa and adhere the same.
Regards,
Team Support
for FRRO, Delhi