Your existing users will have already seen the discobot’s welcome message when they first created their account on your site, so they are not likely going to see your modified text. New users should see it when they first create an account on the site though.
There is nothing built into Discourse that is designed to send a mass email to all users on your site. It would be best to use an external email service to send this type of email.
To notify all users on your site of an important announcement, the recommended Discourse approach is to create either a pinned topic or a banner topic. To do this, create a topic for your announcement, then click the topic’s admin wrench:
Select “Pin Topic” from the menu that appears. Then select the appropriate type of pinned topic from the modal window that opens: