I have a Poison Necro, and I’m using Blackbog’s Sharp.
Would I get more +skill boosting from it if I upgraded it from an Exceptional (Cinquedeas) to an Elite (Fanged Knife)?
Or would it changed from being Blackbog’s Sharp (Unique Cinquedeas) to Fleshripper (Unique Fanged Knife)?
Or would everything on it except the +skills be upgraded?
Basically; What would upgrading Blackbog’s Sharp do, if anything?
I would become a Blackbog’s Sharp Fanged Knife, the base damage would be that from a Fanged Knife, the base requirements will be of a Fanged Knife and all other modifiers will remain the same.
Basically in this case it would go from 30-76 damage, Req Dex 88 and Str 25 to 30-102 damage, Req Dex 86 and Str 42.
So with this one not worth it unless you somehow really need that -2 Dex and can handle the +17 Str.
Some uniques it’s worth it, but most aren’t.
PS: sometimes base speed will also change or when upgrading into for example a Phase Blade it will become Indestructible (in theory you could get a very powerfull rare ethereal indestructible Phase Blade, in theory…)
PPS: Unique armours have max defense +1 when they drop, but the upgraded ones will have a random defense from the base items minimum to maximum, so sometimes you could actually get a lower defense Elite than the Exceptional you upgraded had.
Use this to experiment if you want to see what you can get.
https:/ /maxroll.gg/d2/d2planner/
When you upgrade a unique the only thing that changes is the base item. So it would increase the damage of the dagger. None of the bonuses change. Well, sort of, some of the bonuses re-roll, so if like the +damage has a potential range of +100% to +150% and it currently is 125%, when you upgrade it may go to +135% or +110%.
But Bascially it stays the same. It’s mostly worth it for weapons to increase damage or armor to increase defense. But its not always worth it.
When leveling something like a Barb or Paladin I’ll often upgrade stuff like Normal level uniques to NM level uniques if I don’t find anything better when I get to NM, because the cost of upgrading is pretty low. Going from NM level to Hell level, i.e. Elite, can often make the gear unusable due to increased stat requirements, and is more costly. Still, it’s worth it sometimes.
You will get an Elite Blackbogs Fanged Knife with a new damage roll.
Since the +skill modifiers seem to be fixed??, they should stay the same.
Usually such modifiers (with a range) AFAIK would be rerolled too.
Level and stat requirements will be updated, too.
According to Maxroll.gg this item is considered “trash”. 
https://maxroll.gg/d2/items/valuable-unique-set-items
Thank you for the info, everyone. 
I have a poison dagger nec, I upgraded mine for slightly more physical damage but admittedly pretty miniscule. If you’re trying to get more +skills for like pnova, best bet is either dweb or either magic wand/white runeword as you can get +6 pnova that way.
Magical stats aren’t rerolled when you upgrade items, it’ll still have the exact same ED and whatnot.
The ONLY thing that changes when you upgrade an item is the base item itself, and the only thing that is ever re-rolled is the base defense on armor pieces. The Unique/Set/Rare stats/features always stay the same.