When you upgrade horsemen, you need to have a horse or a war horse in your inventory.
My point is stated above. Allow player to chose the exact horse while upgrading horsemen.
During this upgrade you may lose a valuable horse you need, instead of a horse you'd prefer to give away.
F.e. I buy Desert and Aserai horses in Aserai villages to sell them elsewhere for profit. I can't control, which horse will be lost during such an upgrade and lose profit if the game chooses the horse I need, instead of some other horse I would rather give away.
At first I thought it's irrelevant, but it's not.
UPD. Mod, which solves the problem for the most part. Well, of course sometimes the cheapest horse is not 100% the one you wish do give away, but still. If what I suggested is too complex, than this can solve the problem if implemented in the game.
The mod will prioritize selecting cheaper horses when upgrading cavalry.When upgrading units to cavalry in the base game, it seems that your most expensive horses tend to be consumed first