It can happen after replacing the LCD, but the fan speed should not increase too much.
Try checking the LCD specification if it is the same as the original one (resolution, refresh rate, and panel type).
You can also check CPU/GPU temperature using HWMonitor or HWiNFO while the laptop screen is on and compare it with external monitor mode.
If the temperature is normal, maybe it is just the GPU working differently when driving the internal display.