Noob question concerning 'upgrades'

I was wondering if there was a way to make it so when I have an upgrade finish, it adds a different upgrade to it's place, similar to the way the normal attack upgrades in melee games work (but not exactly).

Basically, I want it to be like this:

Building has two current upgrades for a unit: A and B
Player researches Upgrade A; Upgrade C becomes available
Player researches Upgrade B; Upgrade D becomes available
Player researches Upgrade C; Upgrade E becomes available, and so on and so forth.

I was hoping that there would be more than just the 4 effect options in the Upgrade Editor, but as far as I can tell there isn't and I have far more than 4 upgrades that I need. I've messed around with triggers and the only thing preventing me from finding a simple solution to this is an action that adds an available upgrade to a building.

Basically, I'm wondering if there was a trigger to do this, but if not I can just improvise.

Thanks

Bump

ok well I would say do it this way:

make an upgrade that has lets say 10 levels

then trigger a "Player - Make Level 1 Available for training/construction by Player 1 (Red)"

then have an

event
unit enters playable map area
actions
for each integer a from 1-12 do
if player number owner of triggering unit = intiger A then
set research level of upgrade to 1
unit remove triggering unit
Player - Make Level 2 Available for training/construction by Player[intiger A]

with some modifying ect you can make it possible to put all of the actions into one trigger or however ya want todo it