Yes, it will find the old URL because it's based on the ID.
Same happens with topics too! If you take the current topic as example, it's url is: https://invisioncommunity.com/forums/topic/464437-changing-current-forum-name-effect/
but a not existing name https://invisioncommunity.com/forums/topic/464437-made-up-name/ (e.g. if the topic title changed, the URL will also change) and redirect back to the proper url.