I've replaced steerers on a few forks. It's a pain in the rear, and the next time I do it the cost will be twice what I've done in the past! I don't heat up the fork crown to remove it, as doing so might (probably?) detach the fork blades as well. It's cut, drill, and file, all by hand. New steerer gets silver brazed in, almost all vintage production bikes have fork blades that are bronze brazed, which melts at a signficantly higher temperature than silver based rod, avoiding the fork blade issue. I wouldn't reuse the old steerer, cost of a new one is insignficant compared to the labor cost.

This means that fork better be something special, otherwise you're better off just getting a new replacement fork.