Hello.
Maybe just stop the first "as existing" sketch, and start another over the top.... stop that when done, and then another and another etc. for each design iteration, using the bulbs to control which you have displayed at a given time??? It's a bit clumsy though.
Technically the "save as" solution would work of course.... but in reality its not going to give the functionality I expect you will need for this exercise? It's so important to keep referring back to the "As Existing" drawing frequently when you are working on plans. I'm an architect, and so (I hope) I know exactly what you are trying to do. We are exploring this software, but for small 3D detail elements we need to get manufactured, not for architectural layout work.
I would not say I know this software at all well yet, but for however powerful and intuitive it seams to be, I really can't see it comparing with architectural CAD software for this kind of work? This software is way too sophisticated, most architectural software is pretty simple.
let me know how you get on anyway!