YouTube announced to introduce a change in the URL of the individual video pages. In a post on YouTube API blog, YouTube described the upcoming changes as:

URLs for YouTube video pages will change from the format 

http://www.youtube.com/watch?v=ylLzyHk54Z0 to 

http://www.youtube.com/watch#!v=ylLzyHk54Z0.

That is to say, the ? character in the URL will be replaced with #! characters. Not all users will be directed to the new URLs, and the change will be implemented gradually over time. This change is part of a larger AJAX rewrite of the YouTube web site. The old-format URLs will continue working indefinitely when used for browser navigation, so if your web site has existing links to YouTube video pages, you don't need to worry about them breaking.

Hence, it will not affect the existing links on web pages, as the present URL scheme will continue to work for browsing. Though, this change will certainly affect the tools which use the video URL to extract information about the video. (for example, several YouTube video downloading applications parse the video URL.)