If service is required, and the repair depot sees that the screen isn't original - yes, they'll refuse warranty service.
That's exactly why you keep the original and swap it back before sending it in. Easy 😛
Just purchased a Dell G7 15 (7588) 256GB 8GB memory model and upgraded memory to 16GB and installed additional 1TB SDD. I had the same symptoms as others in the community where boot times went from instant to 3-4 minutes, but only from cold boot. Reboots were instant.
I checked for BIOS updates and saw I was on version 1.3, version 1.8 was released Jan 22, 2019. I updated BIOS and performed multiple cold boots and reboots, Instant boot times once again.
I'd say its fixed.