Could that have been a Deeper Dungeons feature?

No - this is normal. You are probably still on the tutorial levels where the demon spawn are capped at level 10. Once you get further in the game this cap is removed.

Yeah, I believe you can train those demons to dragons in later levels (when you get dragons the normal way)...

That's how it works, as far as I remember.

I can confirm this. The first level you can actually train Demon Spawn up into dragons was Moonbrush Wood, the eigth map if memory serves.

I don't even have the game yet, but I'm waiting for paypal to process my payment. I just recall this quite well :P

Thank you all, I'm relieved (:

Since people here seem to know what they're talking about, I figured I'd ask this here.

I vaguely remember a couple of years back last time I had a chance to play the game that Transfer Creature could be used on Imps. Was this feature added/removed in a patch that this GOG version does/doesn't have? Or am I remembering this completely wrong.

I can also confirm that Demon Spawn turn into dragons beyond level 10, not at level 10.

I remember that that thieves were also changing into knights after level 10.

Wait...people actually let their minions live to reach level 10?!

Bit of thread necromancy here, but yes, imps could be transferred - but I never remember seeing any below level 6-8 being allowed. Of course, I doubt you'd want to transfer any imp lower than that, so it makes sense.

I remember imp transfers being possible too, but was this only on special levels, or only after tutorial levels? Because I tried to transfer a level 10 imp (from the 3rd or 4th tutorial mission, where you have the triple gemstone near DH) but I was unable to... transferred a level 10 warlock instead.