Change Link text with CSS

Use font-size: 0 to hide the original text. To restore the original size use font-size: initial. You won't need to care about the original value.

a.testclass {
  font-size: 0;
}

a.testclass:after {
  content: 'new text';
  font-size: initial;
}

You can hide the original text by using font-size:0; then adding the original font size back to your after:

a.testclass {
  font-size:0;
}

a.testclass:after {
  content: 'new text';
  font-size:16px;         /* original font size */
}
<a class="testclass" href="someurl.com"> CHANGE THIS HERE </a>