[2019] [Feature Request] Please add the ability to start a replay file for an existing save

As the title states. I'm not sure how many other people actually want this, but I'd really love to be able to start a replay file from an existing save file, for example one that I'm 100hours in but forgot to check "enable replay" when I generated. That way, even though I missed the first section of building, I can still record everything in the future (even across updates if I remember to periodically save videos).