I am currently playing an expert early game as Poland. I had been heavily using Avar Nobles as my heavy cavalry and I have several high star generals who are of this unit. As soon as it became the High period, I could no longer produce Avar Nobles. A mounted unit that I cannot remember the name of also disappeared from my production in Khazar. though I didn't use that one so it's not much of a problem.

A few questions:
1) Why did this happen?
2) Is there any way to get the production back?
3) If not, what advice do you have for keeping these generals usable for the rest of the game?

This is really an annoying situation for me since I am currently at war with the Byzantines, the Germans, will probably be attacked by the Papacy soon and the Horde is coming in 20 years and I haven't prepared my armies for them yet. Having these generals be unusable in combat will be incredibly annoying.