The code is not a legal problem, existing installations are generally still legal as installed but your concern for safety is valid. Shop around and try to get bids from a few electricians and see if you can get references. Issues you may encounter is older circuits that are not wired such that Arc Fault breakers will work so that may involve extra work to find neutrals shorted to ground and improper connections between neutrals that may have been OK, if not legal. Whether you actually need 200 amps or not really depends on any added load that was installed but it really does not add much to the bid over staying with 100a.
