<–more–> and formatting the single pages

I’ve got a sizable blog with 400+ posts that have similar problems that I need to correct. Each of the posts have multiple <– more –> tags in there. While it does not effect the view of the content, it would likely complicate what I really want to accomplish.

In the single page views, I want to add in a sponsored advertisement in place of the first <–more–>.

Is there a way to first, remove multiple more tags and second, replace them when calling up the_content()?