If you remove the floor and then sister to a new flat and level floor it will not affect the walls.

Plaster cracking is from jacking the floor (and thus the walls) to re-level things.

Okay, thanks for the response Brickeyee. Just to provide a little more background, there was some work done in the basement previously. Someone supplemented the old center beam of the house with a new beam (looks like a couple 2x8's screwed together) supported by three screw jacks. It's been recommended to me by an engineer that another beam be installed on the other side of the old center beam as well, and then get some identical rough sawn 2x6 lumber as whats there already to sister in. I think three or four of the joists will need to be sistered.

So if I do put up another beam and jack it up just enough to provide support(and not try to level things out), we can remove the old floors, sister in the new lumber level, and then attach the flooring to that, right?

"we can remove the old floors, sister in the new lumber level, and then attach the flooring to that, right?"

I have done it a couple times over the years.

You probably do not even need ti sister with wood as high as what is present.
Just adding some 2x to one upper edge of the old joists may be more than adequate.

While the wood may have sagged from being over-spanned, the new wood may be more than adequate to strengthen the joists so they will not sage any further even if it is smaller than the original joists.

Building up larger timbers with layers of 2x lumber is often more than adequate.

Large pieces of graded lumber can be painfully expensive, and in many cases you end up having to buy larger than you need and then cut them down anyway.

On a limited basis it is also possible in some cases to use hardwood lumber with its often greatly increased strength.

I restored an older house that everyone else was scared of.
The joists looked badly over-spanned, but I noted no sag and very stiff floors.
The joists turned out to be rough cut 2 inch thick oak.

It also explained why all the wiring perpendicular to the joists in the basement was on running boards.
Those old joists could simply not be drilled.

Okay, thanks for the info. The current joists are 2x6 pieces of Walnut, I think. I'm not looking forward to nailing anything at all into them. I do have a contact at a local sawmill, which is why the option of getting hardwood in there was even brought up. But as you astutely pointed out, it is a lot more expensive than standard 2x lumber, and it may be out of my financial reach at this stage of the game. I'm going to do some measuring this evening and see how long they need to be, and then see what the mill wants for the hardwood. I guess that's a good place to start.

If you are removing the subflooring, I'd go with brickeyee's suggestion of tacking on some smaller pieces rather than sistering the whole joist--but have you thought of this:

shim the tops of the old joists with wedges running the length of the joist so that the new shim gives a level surface across the room? You will need to find the high point of the floor, and do measuring so that all the shims come up to that level, but you won't have to try nailing into the old wood--just use construction adhesive.

"shim the tops of the old joists with wedges running the length of the joist so that the new shim gives a level surface across the room?"

If you think sistering is a lot of work, wait till you try shimming.

At least with sistering you can just put a level on the sister, adjust, and fasten it in place.

I do not even shim studs any more.

Add a sister and fasten it in place.

You might have better luck with a nail gun into the old wood, or even screws.

Small holes are not usually to bad in old wood.

Do not use drywall screws.
They are hardened and break before giving under load.
They are not suitable for structural use.