Changing the default or extending a shortcode

In wordpress is it possible to create a plugin that would cancel/ overwrite the default shortcode or extend a built in short code?

Is that even possible?

For example if you paste the URL of a youtube video or vimeo it embeds the video player.

Or the short code is

[vimeo 240975]

??

yes its possible what exactly do you want to override about it? ps. its the embed class located at wp-includes/media.php depending on what you want to do theres a couple of ways to go about modifying it

Basically alerter the code that it’s putting in the post. Change it to HTML5 .