In organizations where you have multiple management groups. And have production environments which need SCOM to be working all the time. Its advisable to have two action accounts in the SCOM environment. When one account's password is near expiry you can switch to another account and change your healthservice to run from that account.
Its a big task to rotate the action accounts on all management groups and this is where my script comes handy. It helps change the default action account on SCOM 2012 and also changes the Data Warehouse Report Deployment Account. The script files are below.
Change the extension to .ps1 after downloading. The usage is given at the top of the scripts.

Om12ActionAccountRotate

Om12ActionAccountpasswordUpdate