<button class="btn">Change layout</button>
<div class="grid">
  <div class="item" id="vt-1"></div>
  <div class="item" id="vt-2"></div>
  <div class="item" id="vt-3"></div>
  <div class="item" id="vt-4"></div>
  <div class="item" id="vt-5"></div>
  <div class="item" id="vt-6"></div>
</div>

const grid = document.querySelector(".grid")
const btn = document.querySelector(".btn")

btn.addEventListener("click", () => {
    document.startViewTransition(_ => grid.classList.toggle("col-count"));
});