<video id="videofilmtv" width="480" height="270">
    <source src="movies/vid1.ogv" id="ogg1" type="video/ogg">
    <source src="movies/vid1.webm" id="webm1" type="video/webm">
    <source src="movies/vid1.mp4" id="mp41" type="video/mp4">
    Your browser does not support HTML5 video.
  </video>

<a href="#" onClick="changevid(A);">

var A = 'vid2.ogv';
var B = 'vid2.webm';
var C = 'vid2.mp4';

function changevid(q) {
    if (changevid(A)) {
        document.getElementById('ogg1').setAttribute('src', A);
        document.getElementById('webm1').setAttribute('src', B);
        document.getElementById('mp41').setAttribute('src', C);
    }
}