i downloaded the game on my iphone when i was using one apple id and then i had to change it due to billing info and now when i try and buy crystals it tells me to delete and redownload the game... but im sure i will lose all the monsters! any ideas?

When you delete the app on your Apple device the game data will not be deleted it is on the AGE servers. As long as your Apple device has not been factory reset you will not loose your game account data.