Changing every URL on a site, or moving it to a new domain, is a lot like moving a business to a new location. Every page has an address, and that address is already stored in Google, in links from other sites, in emails, in ads, and in your customers' bookmarks.
When the move goes badly, the problems show up: broken links that lead to error pages, botched redirects (a redirect is the automatic forwarding from the old address to the new one), and a drop in traffic that can take a while to recover.
Google's own guide to site moves is clear about it: during a move, rankings normally fluctuate while Google recrawls the site, and for a medium-sized site it can take a few weeks or more for Google to start showing the new addresses instead of the old ones. Larger sites take longer.
In other words, you take on real risk in exchange for a signal that, according to Google, weighs almost nothing.