How to connect new trap to old drain system

[amark16]

I'm doing a DIY bathroom renovation and trying to figure out the shower tray plumbing. Please see attached photos

So I removed the old porcelain shower tray which was built into the ground, unscrewed the drain from on top the shower tray, and I'm left with what appears to be a 1 1/4 in thread at the drain entrance.
The drain hole of the new tray will sit approximately 20cm behind the drain on the concrete base.
I will be placing a new resin shower tray with a rising kit so there will be some room below for new plumbing.

My question is, how will I connect the new trap to the drain you see on the first picture? Do I have to remove that so that the drain pipe is exposed? Is there some sort of connection that can screw on to that and somehow reach the new trap which will be 20cm further back?

Apologies if the terminology I'm using is confusing, I live in Cyprus so trying my best to describe the issue!

 

[Geofd]

You will have to open the floor to expose the drain pipe you will need a new strainer assembly,and you may need to move the drain to accommodate the new shower

 

[amark16]

You will have to open the floor to expose the drain pipe you will need a new strainer assembly,and you may need to move the drain to accommodate the new shower

thanks for the reply. Sounds like quite a bit of work. I guess I'll have to do it if its the only way possible.

I was hoping something like the following would be a solution. Oversimplified and not to scale obviously...

 

[GReynolds929]

Don't know code for your location, but here in the US that would not be legal as there would be two traps and no vent.

 

[amark16]

So after doing some more digging this is what I have uncovered (please see photos)
As far as I understand there is a p-trap that is connected to a bigger metal drain pipe that seems to run vertically.
Everything works fine, there are no leaks or anything, so if I can achieve my goal without being too invasive I would much rather do that. Plus,. everything is set in concrete, and this is on the second floor, so I want to avoid something going horribly wrong.

As you can see the old shower tray which I have removed was sitting at the square area whose level is lower than that of the rest of the bathroom.
My goal is to bring everything to the same level, so I'm planning to fill that space with a mud bed and then tile over it. Ideally I would like to install a linear drain. Is this going to be possible without removing the existing p-trap?
The old drain hole has a thread of 1 1/2 in where the drain would screw in. What accessories/fittings do I need? If this is not possible without removing the existing trap, what do I need to do then?