Sketchup would be the best. There is a pretty good learning curve however.

Almost every CAD type software is a learning curve to use . I still like sitting down with a nice fresh piece of paper and a shrrp pencil to do plans for clients You need an interior designer and that should have top of the list before anything was done to the house . You have plans use those to draw the spaces to scale .

Have never used, but since they are free you could test to see which is easier/gives you an overall sense of what might work from an interior design perspective.

Chief Architect has various software programs, and I imagine one of them would work. I rented the Chief Architect Premier version for 2 months at $200 a month (other versions are less expensive, but this one had a lot of features so I just ran with it). I wanted to see 3D designs, as my kitchen designer only provided 2D floor plans and elevations. Worth every penny to see the 3D in IMHO. Really gave you a sense of "is this too big for a particular space" sort of thing.

I tried using other free programs, but they all either had a learning curve that I found frustrating (and I have used many Adobe programs in my life, for example) or did not let me "do" what I wanted (locked you into standard sizes for things, etc.).

https://www.homedesignersoftware.com/chief-architect/

https://www.homedesignersoftware.com/chief-architect/?gclid=Cj0KCQiA35urBhDCARIsAOU7QwkEvEFqG1GvKNc8jinxbhwB7gnho32fgWU8trYWE-gZaOVM7lh62m4aAlOZEALw_wcB

The free version of Floorplanner would work for much of what you describe, but im not sure about uploading your existing info.

Back UP.

Draw and measure the house as it exists. The whole floor plan you are considering "remodeling"

Every wall, passage, window, opening , closet, the what and exact where of everything. Feet and inches. All you need is a tape measure, paper, and a scale ruler. Use 1/4 inch to a foot. Graph paper at that scale is very helpful.

What;s the goal and where? For more space? Where? The kitchen?Bathroom? What?

You begin with what you have now. If you think that sounds low tech? It is. And nothing more than that is necessary.

If you have a set of plans stuffed in a laundry shelf? You can photo and upload those as Jpegs...below.

Sounds like they already have as built drawings and an auto cad floor plan. I haven’t tried all the tools but I like sketchup. Invest some time in learning it. There’s lots of YouTube videos. SketchUp School has good intro videos. Go watch a few and try the free version of the product. You can import your dwg directly and trace it to model the house. Creating a single multi storey model will be a pretty big chore.

You could probably also hire someone to create a full model, and then use that as a starting point for modifications.

On the other hand, it will not be a very fluid process to male major model changes even when you’re really good at the software, and will probably not help with ideation as much as getting some tracing paper and drawing out a bunch of options loosely by hand over the top of a print out. The software slows down your thinking process, while pencil drawing frees up your mind to quickly explore ideas.

I’d personally do both!

I see this is an older question so you have probably already found something, but I used a free app (with upgrades that are very affordable) Cubicasa. https://app.cubi.casa/web