What could be the impact of changing System Account's password and if this same account is used for running every SharePoint Services (CA, search, performancepoint, excel services, etc.) and application pools? I understand this is a bad practice to use System Account for each of these services and application pools but recently I have seen a SharePoint farm using System Account as a one man army.
If you change the password and update the SharePoint with new password, after that make sure all app pools and timer service gets the new password. Update a sync with new password, then you are all set.
I don't see any impact if process properly followed. Worst case scenario would be SharePoint goes down if you locked out the account.