When you change the service account in the SQL Server Configuration Manager, it will assign the required windows permissions to the account.

However, there are two permissions that are not granted, since they are not required - but still very important. One is "Lock pages in memory", which prevents SQL Server from being swapped out in times of memory pressuere. The other is called something like "Perform volume maintenance operations". This permission is needed for Instant File Initialisation. That is, SQL Server can allocate a data file with having to zero it all. (Log files still have to be zeroed though.)