Board Thread:Dragon City Gameplay Questions & Answers/@comment-10724830-20130627192741/@comment-9946604-20130627202710

to the best of my knowledge, you can't reteach an attack.

I don't know if you can 'rebuy' another egg.

The gain/loss numbers aren't a glitch, they roll a random attack for each and show which is stronger. Check the wiki for actual attack values.