To the best of my knowledge, only your "current" tree for each year is saved. It does not save each version of your tree like you can do with wardrobe outfits. If you redo your tree, that is your "current" tree, and your old 2020 tree is not saved. Your 2020 tree will not appear in your "Trees of Lumis Past" page until after Luminaire 2020 is over.