Peasant to Footman

How would I make it so i could click on an icon in a selected peasant, that permanently turns him into a footman at the cost of 50 gold?

Modify the "Destroyer Form" ability of the undead, and make the footman cost 50 gold.

Another way to do this is to go into the unit under Stats- is a building. check this. then go to TechTree- upgrades to and add footman. Then go back and un-check Stats- is a building turning it back into a unit. Now the footman will show up with the rest of the unit's commands. You then can change the build time, cost, and icon location of the footman to effect the peasant and any other unit that can upgrade/Train the footman. (of course this changes the data for ALL that can upgrade/Train the footman)

There is a rather peculiar problem with the Destroyer Form ability:

Say you have 4 peasants that have an "Upgrade into Footman" ability based off Destroyer Form, with a cost of 50 gold units. You have your 4 peasants selected and are about to click the aforementioned ability icon.

Say you have the following scenarios:

1. You have 200 gold units
2. You have 0 gold units
3. You have 50 gold units

In the scenario number 1, the 200 gold units would be deducted, and all your units will upgrade into footmen.

In the scenario number 2, you would just get an error message reporting that there is not enough gold, and no peasant would morph|upgrade.

And last but not least, in the scenario number 3, guess what happens? if you guessed that only 1 peasant would turn into a footman, well... you guessed wrong. All your peasants will turn into footmen, at the mere cost of 50 gold units.

What does this mean? That the resources are evaluated before the ability is casted, but they are not re-checked each time an unit casts it. So the minimum required to morph a single unit can upgrade all your selected units.

This leaves us with the Is_A_Building check on the object to show the upgrades into fields. Unfortunately, this has the presentational side effect that it shows an "Upgrade Complete" notification everytime you cast it. If you intent that your player sould do this somewhat frequently, it might be annoying, specially when you can't tell apart if a building or an unit was upgraded.

Good point i had not thought of the "upgrade complete" thing.

This may have the same problem as Destroyer Form, but you could try using Convert Gold to Lumber and replacing the unit with a trigger.