confluence.css

.hover-gallery {
  position: relative;
}

.hover-gallery:hover .confluence-embedded-file-wrapper {
  transition: visibility 1s ease-out;
}

.hover-gallery:before {
  content: 'After migration';
  position: absolute;
  top: 0;
  left: 50px;
  font-size: 45px;
  color: red;
  z-index: 1;
}
.hover-gallery:hover:before {
  content: 'Before migration';
}

.hover-gallery .confluence-embedded-file-wrapper:nth-child(2) {
  visibility: hidden;
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}

.hover-gallery:hover .confluence-embedded-file-wrapper:first-child {
  visibility: hidden;
}

.hover-gallery:hover .confluence-embedded-file-wrapper:nth-child(2) {
  visibility: visible;
}