The GPU is replaceable.  
I recently bought a Del Precision 7550, it is almost perfect, my only worries rely on the GPU, and I plan to replace it when I get enough cash. My version has a Quadro T2000, which was the only one available, if someday I decide to replace it with a more powerful RTX card, could I do it easily or is it welded into the motherboard? 
Is the 7550 graphic card upgradable?