First: the total cost of replacing the current flag is unknowable. Every single government department and organisation, including every private business that uses the flag in any part of their collateral will need to change all of their collateral across print, digital, signage and whatever else to match the new flag. Especially for businesses, taking the route of driver licence cards (and eventually replacing them all) won’t work. For the most part, the country will face a big scramble to update as much as possible as soon as possible. But even then, the process will take several years and a total cost could reach into several hundreds of millions of dollars.
Second: do you have any proof that a new flag will bring a sudden influx of tourism and trade? I find it hard to believe that tourists debating a visit to New Zealand will base their decision on our flag design. And the same could be said for business. How is a new flag design, specifically, going to bring in “probably BILLIONS”? Do you have any supporting data for this?