Following comes from Senior Support Engineer Nitish Nagpal.
I have seen scenarios where TFS Engineers suggest changing the startup account of SQL Service(s) but often do not provide full instructions for doing it. As a result, customer will often go to Services.msc and changes startup account there, which can sometime have adverse effects. I just worked on a case where a customer changed this account from Domain to Local for SSAS in Services.msc which caused corruption in SQL.
The best way to change the startup account for SQL is as follows:- Go to SQL Configuration Manager (Start – Program ->SQL Server ->Configuration Tool ->Configuration Manager)- Go to Service (SSAS / Engine) ->Double Click on it -> Log On (Provide Services Account) ->Apply
Why use SQL Configuration Manager instead of doing this directly in Services.msc? SQL Engine / AS perform some encryption using the service account, so when we change it from configuration manager it decrypts information using the old account and re-encrypts it with new the new startup account. Services.msc isn’t aware of any of this, so it’s best to avoid it for this type of change.
I am always fixing issues where admins have changed the passwords using the services tool.. I always get the "I changed the password of the account now it dosn't work" First thing I tell them is use the SQL Tools and try it again.