SharePoint 2010 has a great new feature for setting your service account passwords for when your AD team has locked down the password policies. In Central Administration you can click Security->Manage service accounts links, and after selecting the account, you can then set the passwords for these accounts from central administration. You may get an Access Denied error via the Microsoft.SharePoint.Win32.SPNetApi32.NetUserChangePassword method. This is very simply calling the C++ Netapi32.dll to change the password. Reference this for more information:
In doing more research, any accounts that have "Do not allow user to change password" will fail to update the password. The actual changing of the password is done as if it is the user changing it. You should also watch out for the "Accounts can only change once per day setting", this would keep you from changing your password more than once in a single day.