[NAME]  Please press the blue Accept as Solution button below if the post answers your question. There is no issue in changing cpu but take care when replacing the thermal paste. Warranty is only supported on the cpu that the system shipped with. Other than that I do not find any issues.
ATM, I have two 3640 towers. Specs no 1: 64GB of DDR4-2666 RAM ECC, CPU = Xeon W-1250P. Specs no 2: 16GB of DDR4-2933 RAM Non-ECC, CPU = Xeon W-1270P. I decided to sell Dell No 2, but want to keep the faster CPU. So, my plan is to swap the CPUs. I know that the RAM will slow down to 2666 (machine 2) because the xeon w-1250 won't support DDR4-2933, and the ECC won't be faster than 2666. I take that. I hope the rest will be fine and both computers will run smoothy. I upgraded CPUs in the past (Mac Pro 3.1 to 5.1, HP workstations), so I know what to do. But I want to be sure, that the swap between my Dells is okay. Thanks for a confirmation or disagreement.  
CPU swap between two Dell 3640