How to upgrade a form renderer you are afraid of
Renderer upgrades get deferred for years because the blast radius is unknown. Making it known is most of the work.
Re-render everything, then compare
Run the whole estate through both engines and diff the output. The list of forms that actually changed is usually far shorter than the fear suggests, and it is a list you can staff.
Define equivalence before you look at the diffs
Otherwise every anti-aliasing difference becomes a debate. Field values, page count, text runs and tagged reading order are a defensible bar; pixel identity is not.
Keep the comparison as a permanent gate
The value is not the one-off upgrade, it is that the next upgrade is a routine run rather than a project.