How to do it?

You should have upgraded the centaurs, not the elves! And crashes will definitely happen, it's Wog after all.

But centaurs can't be upgraded((((( elves can be upgraded into centaur-elves (Sylvian centaur, I think).
Another thing in Wog, I set it up so that the Capitol can be developed to generate 7000 gold per day. The upgrade function appeared, gold and mithril are deducted upon confirmation of the upgrade, but the daily income doesn't increase(((