Upgrade the renderer without holding your breath
Re-render the whole estate on the new engine, compare every output against the current one, and get a list of the forms that actually changed.
The upgrade nobody schedules
Renderer upgrades get deferred for years because the blast radius is unknown. Running the estate through both engines and diffing the output turns that unknown into a list, usually a short one.
Pixel equality is the wrong bar
Agree what "the same" means before you start — field values, page count, text runs, tagged reading order — and make it a test that runs on every revision from then on.
Start with an estate assessment
Point YakForms at a copy of your repository. You get an inventory, a dependency graph and a validation report for the whole estate — before you commit to anything.