Upgrade should be fine. HOWEVER will not be warrantied by Dell. Dell only warranties what it shipped with.  
I was thinking about getting a i5-8400 and since the motherboard is compatible with Intel 300 Series Chips, I was thinking that I was able to get a 8th gen, but I have no idea. If someone can let me know if I can use a i5-8400 on a dell g5 5090. Right now, I have a i3-9100f and want to upgrade.
G5 5090 8th gen CPU upgrade?