I have a problem with my bathroom. I recently had someone come and switch the door from the kitchen side to the other side while redoing my kitchen. He did a great job and the kitchen looks great. That being said he used new studs to re-frame the wall and those studs are smaller than the 2x4's used 100 years ago on the original bathroom. I do not know how to correct this issue while dry-walling.... Any suggestions would be great! Thank you i have provided a picture.
The furring strips are fine but usually a pain to get right and could have some long-term issues. If someone tries to hang something and they don't hit the furring strip they will make the drywall concave and possibly deform.
I am thinking the older 2x4s were actually 2" by 4". Easiest thing to do is double up your drywall in that spot. You may need either 3/8" or 1/4" drywall to make it even. This is usually cheaper, easier, and better long-term.
Depending upon the spacing, you could just go ahead and drywall over the old studs, or fur out the new studs (add a 1/2" or so) to the new studs if they are too far from existing studs. The entire wall will need to be in the same plane. As others suggested, be sure to pay special attention to keeping the drywall flush with the door jamb and well supported or it will make trimming it out difficult.
The easiest way I have found is through the use of these cardboard furring strips.
They're 1/16" thick, so you can stack them together for wider spaces, and you can also peel them apart to make them thinner. I've used them for everything from furring out studs to leveling cabinets and cabinet drawer mechanisms.
I would install some furring strips to the new studs to make them flush with the old studs. You will probably not get an exact match right away, so you should expect to run your furring strips through a table saw a few times.
Also keep in mind that you do not need perfection here. You just need to support the drywall at 16-24" intervals so it installs cleanly. Sometimes you can solve problems like that with some wood scrap, or some old wooden paint stirrers...
This happens all the time. Easiest solution is to get a sheet of hardboard that is ⅛" thick. Rip it down to 1½" strips and put that on top of the low studs. Much easier than ripping wood down and you can stack them up if you need more than ⅛. You can cut them to length by simply snapping them by hand and it'll only take a few minutes. If you're feeling really ambitious you can even glue them to the studs with spray glue. Make sure your drywall screws are long enough to go to the framing and not just the shims.
What you need to do is put a layer of 1/4" drywall over the new framing and then put the new drywall over and that will make your wall straight, if you still have a gap between the levels use some construction adhesive around the edges and that will give you the strength that you need around the door frame.
Fur the wall out. Now a word of caution... Don't make the mistake of furring out the wall to much. If you do this you will run into the problem of the drywall pushing out farther then your door jamb which then will lead you into the problem of your trim having a gap between the trim and the jam. If this happens you will end up having to add some type of caulk to fill that gap... Which if you were planning on staining not painting that obviously is a problem. If your drywall isn't flush to the door jamb it is much more desirable for the jam to be out past the drywall. The reason being is that it is easy to feather out some mud to make the drywall flush with the jamb.
If you have a table saw you could easily rip the proper width of furring strip. If you don't have that, I've even stapled shims or even paint sticks to the studs to get the job done. Best wishes buddy. You can do it!