So after I finally got onto the last platform using the crouch technique, I cannot 'change' the lamp? If have the carved helidium, but there is no old lamp to replace? I tried using the action (e) button while standing in front of the light, but nothing happens.

I have attached a screenshot. Is far is I've read in the walkthroughs there is supposed to be another lamp to replace, right?

Any suggestions?

I solved it.

The old light was still there, but it was not visible (see screenshot in OP), and since I was holding the new light, I could not take the old one out.

Solution: put new light back in your backpack, stand in front of the light fixture and pick it up (although you can't see it). Put the old light in backpack, then install new light.

Hope this helps someone

LeonMe, that solved it indeed, thank you.

Funny thing is, in the first few minutes in Cyana, just before or after getting rid of the soldiers, I looked out to the boat light from high up, near the Samaz's place. I even looked out with the EVD. And I could see the burned out helidium !

Only later could I not see it anymore... even though as it turns out, it was right there and waiting to be removed.