some Lizardmen suggestion for ya !
Kroxigors turn into Sacred Kroxigors
Ancient Stegadon turn into Ancient Stegadon (Engine of the Gods)

Upgrading Demigryphs/Necropolis Knights to the Halberd Variant wouldn't go amiss.

Also, upgrading Pegasus Knights to Royal Pegasus Knights might make more sense than Royal Pegasus-to-Hippogryph

I also think being able to quickly upgrade Empire Archers to Crossbowmen (And possibly Crossbowmen to Handgunners?) would be nice alongside huntsmen, since I see crossbows as being a more standard progression route in your normal game. Archers are just a very very low-tier unit that I imagine a player would want to get rid of pretty quickly so they can get to their more standard units, rather than the more specialised huntsmen
It might be interesting to upgrade Mortars to War Wagons (Mortars)
Maybe you could try upgrading Free Company to Pistoliers? Since they're both pistol-units?

I don't think I like how upgrading Skeleton Warriors to Tomb Guard requires 2 steps, while Skeleton Spears to Halberds requires only one
Another TK thing would be upgrading Chariots to Chariots with bows, as well as Ushabti to Ushabti with Greatbows.
Do Tomb King upgrades bypass unit caps? Because that might have a weird effect on the Tombking Playstyle.

It also strikes me as quite odd that Chaos Warriors have to go through Halberds before they can become Chosen.

You could probably have an upgrade from White Lions of Chrace to Swordmasters of Hoeth (dunno if it's loreful, but mechanically, swordmasters are a straight upgrade)
Similarly, an Eagle becoming a Pheonix could be cool (But I don't know much about pheonix biology, ahah.)
Maybe Archers (light armour) or Lothern Sea-Guard could, at a very high rank, upgrade to Sisters of Avelorn?

I think going from skink cohort, to Javelins (rank 4?) to skirmishers is pretty logical, possibly branching to Red Crests at one stage.
Also, Feral bastiladon-to-bastiladon-with-a-thing-on-it's-back would be good
Also possibly Cold-One Rider to Horned one at a higher rank

Do you want to use custom units also or vanilla only?

Also what about upgrading heroes, for example a vampire hero into a vargulf?

Custom units are fine, it'd be nice if you can tell me the actual land/main unit keys involved (they should be the same), otherwise I have to go dig them up myself, and who knows when I can get around to that.

I don't think it's worth doing heroes.

I feel like swords and halberds are too big a jump, same with bows and crossbows, they're completely different weapons. I also don't wanna jump races (Red Crested Skinks) or genders (Sister of Avelorn). White Lions of Chrace to Swordmasters of Hoeth is also horribly anti-lore. You don't turn into a Swordmaster of Hoeth or a Phoenix Guard, it's a more involved thing in the lore.

A suggestion:
Glade guards>Deepwood scouts>Waywatchers
Glade guards with magical arrows>Deepwood scouts with magicl arrows

Thnx for the update to the dwarf progression! It's perfect.

It's fair that a white lion wouldn't be able to become a swordmaster, though maybe Spearmen could eventually train to the point of being pheonix guard? I'm not sure quite what the process -there- is, but I can imagine them beginning their career as Spearmen before they do whatever it is you need to do to become pheonix guard, I dunno
Alternatively, getting to a high enough rank as a spear-elf might get you to the point of learning to shoot a bow and become a Sea-guard

Would there be any way of making the cost of upgrading a unit depend on the unit type? Upgrading from Spears to Spears-with-shields seems somewhat pointless when it's cheaper to simply disband the unit and recruit a new one at full health.

Great job with this! I would love to see an SFO submod that allows flaggelents to upgrade into sigmarite disciples and wolfkin into teutogen guard. Etc. There are quite a few good possibilities in SFO for it.

Cold One Riders to Horned Ones
Feral Bastiladons to Ark of Sotek/Revivification Crystal/Solar Engine
Feral Stegadon to Stegadon
Stegadon to Ancient Stegadon
Ancient Stegadon to Engine of the Gods

For bretonnia would it be possible for Peasent mobs to be upgradable to both swords and Spearmen?

And also for Swordmen to go to Battle Pilgrims then onto Foot squires?

Some Suggestions:
Vampire Counts:
Black Knights>Blood Knights
Zombies>Skeleton Warriors (meat sloughs off eventually?)
Skeleton Warriors>Skeleton Spearmen
Cairn Wraiths>Hexwraiths

Vampire Coast:
Zombie Pirate Deckhands Mob>Zombie Pirate Deckhands Mob (Polearms)
Depth Guard > Depth Guard (Polearms)
Zombie Pirate Gunnery Mob > Zombie Pirate Gunnery Mob (Handguns)/ZPGM (Bombers)/ZPGM (Hand Cannons) > Deck Gunners
Rotting Prometheans > Rotting Promethans Gunnery Mob
Deck Droppers > Deck Droppers (Bombers) / Deck Droppers (Handguns)

Since the vampiric factions seem to be controlling these corpses more or less directly, I figure their actual weapon proficiency is a ... dodgy subject as far as zombies and skeletons are concerned anyway.

Zombies probably shouldn't upgrade to Depth Guard, seems weird for them to become vampires all of a sudden. Zombies > Crypt Ghouls also occured to me, but I think I saw mention that WH ghouls aren't entirely dead somewhere? Not really sure on that front.

Could you make it so Lizardmen units could upgrade into their blessed variants? that would be awesome!

Hi, can u add more restrictions to it? It definitely make the gameplay more immersive, somehow less challenging.

Can you be more specific? Whom were you playing as, what you found the problem units were etc.