Built the Rogue, Warrior and Thief Guilds, but I have no idea how to train the advanced units. I have Catapults, Clerics and Mages just fine.
I also believe that when you upgrade a unit it then suddenly requires very little experience to gain a level. After upgrading I seem to gain a level shortly afterwards.
That's just coincidence. There is a blue bar at the bottom of the unit portrait that indicates progress to the next level, and if you mouse over it it'll say how much experience it has and how much it needs to gain a level. Upgrading a unit doesn't have any visible effect on experience.
I checked with a level 1 unit, which takes 20 to level the first time anyway. Apparently it's resetting the level curve back to level one when you upgrade, and testing with a level 1 unit wouldn't reveal this. I agree that it's probably a bug, but it would let you babysit a single unit for a few levels, upgrade to an advanced form for a few levels, and then upgrade again to a temple unit to get the most out of it. It takes a lot of paying attention, but I guess if you want to focus on a couple of units with a dedicated training regimen, it could be a fun exploit. Still probably ought to be fixed by the time multiplayer rolls around, though.It's not coincidence. I checked it and after upgrading the required experience for next level is reduced to 20 points. Looks like a bug.
I checked with a level 1 unit, which takes 20 to level the first time anyway. Apparently it's resetting the level curve back to level one when you upgrade, and testing with a level 1 unit wouldn't reveal this. I agree that it's probably a bug, but it would let you babysit a single unit for a few levels, upgrade to an advanced form for a few levels, and then upgrade again to a temple unit to get the most out of it. It takes a lot of paying attention, but I guess if you want to focus on a couple of units with a dedicated training regimen, it could be a fun exploit. Still probably ought to be fixed by the time multiplayer rolls around, though.
There's some large advantages of upgrading trained units rather than just buying them, for example some temple units change unit type when they upgrade giving them different perks, so you can have the best perks of multiple unit types.