Hi Whitey, It's hard to know. Did they create a single redirect for 15k+ pages or break it up or even go so far as to create redirects for every page? Rewrite rules can be created but it depends upon what they're starting with.
They built the 15k pages and put redirects in place from old site to new site. That site has not gone live yet. They then received a recommendation to alter the url's, because we don't want multiple redirects occurring in the future for the sake of seo stability.
It looks like it's down to a couple of days work, but they weren't happy that it wasn't communicated at the beginning of the project.