Simply duplicate it. I know you said you don't want to duplicate it? Why not? As duplicating - and assigning a new new name is same as using save as - with old file name and new file name. I do this with all my pages and numbers files when I want to save a file at a certain point to keep. often for protection so not too much is lost.
Or to alter a quote or invoice for a similar job. Keep old, make new client file name with old information, simple.
It is also the same number of steps -1 - it is just at beginning of edit, not at end. This is simple to create 2 similar files of work progression. and command S, save new current changes as you do your work to the new file name.
It is also best for photo alteration also, as I saw a previous post about auto save, when save as removed and he lost old file. Duplicate it first, add new name, eg: photonameV2, then make changes, this way you always have original file to go back to anytime you want.