It's pretty obvious that if an element changes from top: anchor(--foo top); to top: anchor(--foo bottom), that's a computed-value change that will trigger a transition on 'top'.

But what happens when the value is top: anchor(--foo top); the whole time, but you change which element has anchor-name: --foo;? @jh3y has an example of this using the popover API, where they change the popover attribute, altering which element is used as the "implicit anchor element" without actually touching the CSS. They'd like the abspos to smoothly transition to its new position.

I think this gets solved appropriately with the exact same solution as #8180 - if the tree-scoped reference is resolved as part of the computed value, then having it change underneath you counts as a computed-value change, thus triggering a transition, and the start and end values are (non-textually) distinct, referring to the old and new anchor, so you can meaningfully transition between them.