If you contract a disease few years after buying the policy, it is not mandatory to disclose it. But, if you want to increase the sum assured, then premiums could go up sharply
As for the query on income tax, you are allowed to claim a deduction of up to Rs 25,000 in each financial year, for the medical insurance premium that you pay
Medical expenses getting deducted due to limits/cappings under group health policy can be claimed under individual health policy
It doubles cover at no extra cost in case existing sum insured is exhausted, and is also cheaper
CI cover is useful if you have liabilities; top-up plans cover all diseases