Hello... I am trying to gradually work my way up on some home projects and I think this one might be too much for me. I want to remodel the bathroom and make some more room... this requires switching the toilet to the opposite corner and moving the shower out of that spot and turning it 90 degrees to the back wall. From previous posts, it appears that I can use the existing toilet drain for the shower, but how do I use the old drainage from the shower for the new toilet location. This bathroom is on the 3rd floor. Is there a way I could use the dead space under the stairwell, which is nearby?

You can drain a shower into the old toilet drain but you can't drain a toilet into an old shower pipe which is likely 2" . You need at least a 3" drain for the toilet. You might get lucky and find that taking up some flooring will expose the 2" pipe enough so you can tear it out and run 3" to the main drain. Did you know that there is already a K.I.S.S. person on this site?

Bob's correct.
You can drain a shower into the old toilet drain but you can't drain a toilet into an old shower
If this were my job I would tear out the old plumbing with the exception of the vent pipe and injstall my bathroom group like this. Toilet connects to the stack vent. Lavatory connects to toilet drain and runs a vent line to the old vent going out the roof.The toilet wet vents through the lavatory vent and the tub/shower connects to the lavatory drain and is wet vented by it. This is a normal rough in and is acceptable both by local and state codes and also The Standard Plumbing Code Book in 90 percent of the country. Check your local codes to make sure you're not in the excluded 10 percent.. More questions? I'm as close as a click, Tom