Well...I believe you could add some conditions to your tiggers that would determine the type of building you have and then spawn the correct creatures.
Ok, so do the following:
Code:
Event - Unit - YOURMAINBASE Finishes an upgrade
Condition - (Researched tech-type) Equal to NECROPOLIS
Action - Set Undead == 1
Set Human == 0
Set NightElf == 0
Set Orc == 0
Ok so let me explain. The Undead, Human, NightElf and Orc are "real" variables. They will determine the type of bulding you chose(in footmen frenzy there are undead, orc, human, nightelf, etc...) and the number will determine what level the building is, so for example, if you have a CULTOFTHEDAMNED which would be a level 3 undead building and you want to update it to a level 4 Nightelf building you will do the following:
Code:
Event - Unit - YOURMAINBASE Finishes an upgrade
Condition - (Researched tech-type) Equal to CHIMERAROST
Action - Set Undead == 0
Set Human == 0
Set NightElf == 4
Set Orc == 0
I assumed that the CHIMERAROST is the level 4 night elf building.
Ok, now that we have our trigger that will tell us what is your buildings level/type we need to make spawn triggers:
Code:
Event - Every 10 seconds
Condition - NightElf Equal to 4.00
Action - Unit - Create 2 Chimeras for player 1 at center of YOURREGION facing default building degrees
Now, just change the conditions and the units you want to be spawned and things will work smothe.
There is a problem however. You will need to make a variable that is called YOURMAINBASE and will store the building that your main base is currently(necropolis, barracks, cult of the damned, etc.).
This isn't a real problem, but I am not sure how you would do this exactly since I rarely handle unit variables, but if you are good with the WE you will find a way to do this easily.
Also note that this might not be the best way, but it is the only one I can think of at this moment. Further more, you should find an unprotected version of footmen wars/frenzy and study it in the WE.
Hope this helps,
Kafana