Town Hall Upgrades

Hello! I've been working on making extra races, and I'm not sure how to make an upgradable building.

For example, a Town Hall.

My idea so far includes a Unit being used as the Upgrade, and having you "train" it. Then, when the unit is trained, the unit is Removed and the building is Replaced with the new one.

However, while this works, there are two problems:

#1) If there is a second upgrade, then anything that requires the first upgrade to be built will stop working. For example, a custom-made "Keep", when upgraded to a "Castle", will prevent you from building an "Arcane Sanctum".

#2) You can train multiple upgrades in a row... Making a waste of resources and something that doesn't look pretty at all...

Now, I think I've come up with solutions for these, but I'm not sure if they'll work very well...

#1) Have each "Town Hall" come with an invisible, invulnerable, itsy-bitsy-teeny-weeny unit. This unit will be the one that is required for the other buildings. When the Town Hall upgrades, the unit remains. However, when the Town Hall is destroyed, the unit is destroyed. This fixes the requirements problem, but it adds a unit... and it will be visible to the player it belongs to. I'd have to make a trigger to automatically deselect this unit if it's selected... Complications -_-

#2) I'm wondering if there's a way to see which Unit a building is Training before it's trained. I can do the upgrade by saying "Unit - Unit Finishes Training" with the condition "Trained Unit = Upgrade to Keep". However, I can't say: "Unit - Unit Begins Training" with the condition "Training Unit = Upgrade to Keep".
If I could do that, I'd be able to disable the ability to make that upgrade. Of course, this also has complications. For starters, if the building is destroyed or the upgrade cancelled, you can't build it again. Second of all, you can't do it again.. at all. So, I'm wondering what the workaround for all that is... ~_~ Argh.

Any help would be appreciated :)

Of course, I could always just use a variable to see what race you are, and base all the Town Halls on the same Town Hall, and make it check to see if you have a Keep - and if you do, to change it to the "Keep" instead.
This is really not wanted, though, because the "Upgrade" Button will still remain as "Upgrade to Keep" with the wrong picture...
So, training a unit seems the best option, despite..

*sighs* ~_~

Or you could do something original and make your race unique... not following the standard 2 tiers, and making more than one builder unit. Here is how one o the races for my mod is structured:

Hall-->Builder unit
Supply-->Gives 8 supply
Barracks-->Basic 3 fighting units
Lumber-->Lumber return place
Siege-->2nd Builder unit for static D
Magician-->Standard 2 Magic Users
Aviery-->Standard 2 air units per race
Small Temple-->Trains heroes and a 3rd builder unit

2nd Builder Unit:
Siege Tower: Long range siege damage
Pit Tower: Short range, very high damage piercing
Mine: Construct a land mine
Spike Pit: Slow attacking melee range tower to emulate the Dungeon tileset spike pit.

3rd Builder Unit: (Construct Temples which allow higher powered buildings)
Temple of Marius-->Allows the Magician type building to be constructed
Temple of Venuire-->Allows Aviery type building to be constructed
Grand Temple-->Trains tank melee unit, and a specialized ranged unit.
Temple of the Gods-->Has Fountain health and mana regen, but is insanely expensive.

This race structure is a large breakoff of what Blizzard uses as its standard, but it still works and is quite balanced. Go for something original, not just slightly modified building structure.

The thing is, all four races have the 3-tier System.

Since I will be including the "new" races with the four old ones, I want them to balance easily...

Besides, the 3-tier scheme seems to be one that Blizzard wanted.

I have been thinking about races that don't have the upgraded town halls, but, in any case, I'd like upgrading town halls nontheless.

What about Chaos Orcs? You'd want those the same as ordinary orcs - but you can't, without an Upgradable Town Hall. Same with Corrupt Night Elves, for the most part, and High Elves.