1. I doubt you would get the Plasson coupling between the elbow you want to alter and the tee going down to the right of the joist.
2. The Plasson coupling is 54 mm in diameter, so probably wouldn't fit your space anyway.
3. If there isn't much clearance between the two steel pipes where they cross, I don't see how you are going to be able to cross over the underneath steel pipe in the space between ceiling and top of joist.
4. I suspect the way to do it is:
4a. Water off.
4b. Cut the pipe closest top the wall just to the left of the joist.
4c. Unscrew the remnant of the pipe from the tee to the right of the joist.
4d. Use a 3/4" x 22mm male compression iron screwed into the tee to the right of the joist. This will need to be sealed with PTFE, Loctite 55 or a sealant such as Loctite 577 or Flomasta Pipe Seal. Make sure all threads are clean and grease free, and if using Loctite 55, gently rough up the male 3/4" threads on the male iron.
4e. Use 22 mm copper to extend the pipe beyond the one it currently crosses.
4f. If possible, re-arrange the route / destination of the existing copper pipes so that no cross over is required.
4g. If not possible to avoid crossover, either make or buy a suitable cross over. (e.g. BES 9201)
5. No special tools required for the above, but it's not straightforward. There might be difficulties unscrewing the steel pipe from the tee, and if that gets disturbed, there would be further work required.
6. The above assumes 3/4" nominal bore steel pipe and 22 mm copper. Measure before proceeding.
