Upgrading Knight to Cavalry with 0 Horses

[FeiLing]

I was able to upgrade this Knight to a Cavalry without having additional Horses.

(Of course the total number of horses needed to not get a combat penalty doesn't change.
I am 120% sure though that you do need at least 1 additional resource to upgrade a unit, even
if the current unit uses the same resource as the upgraded unit. I have seen it work like this
countless times before and also just in my previous game with a Swordsman not being able to
upgrade into a Longswordsman when I was at 0 Iron and in home territory with enough gold.)

A savegame (1.0.1.511): NoHorses.Civ5Save
(requires the Wonders of the Ancient World DLC.)

 

[snarzberry]

Don't know about that one, there must have been some other reason you couldn't upgrade the sword. You don't need additional resources to upgrade units. Or, at least I'm 130% sure that's right. Haven't you had a 6 iron deposit and upgraded 6 units to swords and then longswords?

 

[FeiLing]

Seems like you're right. One doesn't need 1 extra resource to upgrade units.
However I can show you where my confusion comes from:

When you are inside your own territory it looks like in the above screenshot. When you are outside of your own territory it looks like this:

Quite misleading imho. It does make sense though for units that upgrade for e.g. from being oil-based to being aluminium-based.

 

[headcase]

The tooltip is the bug; you don't need 1 horses (that's a 2nd bug, it's 1 horse*) to upgrade the unit. You needed one horse for the knight and now you need the same horse for the cavalry, therefore none (additional) are needed to upgrade and the game should be quiet about it.

* Technically one horse represents many horses but a knight unit is called "knight" not "knights".

In your example where it goes from oil to aluminum, it should say:
You need 1 aluminum to upgrade this unit
You will receive 1 oil upon upgrading this unit